My recommendation is that you hire someone to do this or help you do this.

If you want to do this on your own, start by looking at dnndocs.com for the appropriate sections. You'll want to find the "suggested upgrade path," too.

Basically, the flow is to get the next upgrade package from the DNN archive at Github, unblock and copy the files into your DNN root directory, and reload the website. That likely will trigger the upgrade process, unless your web.config file has been modified to prevent it. In that case, you'll need to start the upgrade manually.

You will want to test at every stage to verify that the upgrade process worked. YOU WILL WANT TO MAKE A COMPLETE BACKUP AFTER EVERY STAGE!!! This will save you hours of work in the event that an upgrade goes awry.

At certain stages, you will need to deal with "breaking changes." This may mean upgrading modules or accounting changes to DNN's menu system, or other items.

You can always ask for help here for specific issues.

I think that there are some videos around that explain the upgrade process.

But, go back and read the first sentence in this reply. It's likely to save you a lot of time and even some money.

i followed as per the upgrade shared path.
i have done as below.
1). copied all the 5.6.0.8 upgrade package to existing dnn site.
2). accessed the http://localhost/site1/In...ll.aspx?mode=upgrade or any other page.
it is redirecting to 404 custom page. it seems authentications issue. the problem is the upgrade page/any page is not allowing me to login . How can i start the upgrade process?How to check the dnn logs to identify the exact error?

Let's go back to the site before you copied upgrade files.

What is the URL that you use to access the site? Does the site work, and can you login?

Yeah, i was able to login to site with super user ->host .
it seems we are using custom authentication.
Atleast the upgrade page should ask me to login first, it is not happening now. instead it is redirecting to custom 404 error page.

url is http://localhost/site1

what happens if you just go to http://localhost ?

localhost/127.0.0.2 is the local IIS. I deployed dnn site in site1 in IIS,  I am browsing dnn site using http://localhost/site1. when i browse this stie. login page will open first .after authenticate  default page will be displayed. 

after replacement of upgraded package.not able to browse the upgrde page, gettting internal server errror. 

I found those errors in  sitelog .

AssemblyVersion5.6.8
PortalID-1
PortalName
UserID-1
UserName
ActiveTabID-1
ActiveTabNameRawURL
/Site1/Install/Install.aspx?mode=upgradeAbsoluteURL/Site1/Install/Install.aspxAbsoluteURLReferrerUserAgentMozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like GeckoDefaultDataProviderDotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProviderExceptionGUIDff9ed1a8-7c13-4b83-b2c8-a741a2a90aedInnerExceptionSearchTabIDFileNameFileLineNumber0FileColumnNumber0MethodSystem.Data.ProviderBase.FieldNameLookup.GetOrdinalStackTraceMessageSystem.IndexOutOfRangeException: SearchTabID
   at System.Data.ProviderBase.FieldNameLookup.GetOrdinal(String fieldName)
   at System.Data.SqlClient.SqlDataReader.GetOrdinal(String name)
   at System.Data.SqlClient.SqlDataReader.get_Item(String name)
   at DotNetNuke.Entities.Portals.PortalInfo.Fill(IDataReader dr)
   at DotNetNuke.Common.Utilities.CBO.FillObjectFromReader(Object objObject, IDataReader dr)
   Source
 

FYI, upgrade from 5.5.0.653 to 5.06.08 didn't work as per upgarde shared path.

But i didn't face any issues when i upgrade from 5.5.0.653 to 06.02.08. everything got success.

I did upgrades as below
5.5.0.-> 6.2.8
6.2.8->6.29
6.2.9->7.0.0
7.0.0->7.0.6
7.0.6->7.2.3

I got the error when I upgrading from 7.0.6 to 7.3.4

00:00:40.863 - Converting old Folders to new format. Total: 2 [Remaining: 2]
00:00:40.883 - Conversion of old Folders Completed. Total Converted: 2
Error!
00:00:41.580 - Application Upgrades : 07.01.01 Error!
00:00:41.975 - Application Upgrades : 07.01.02 Success
00:00:42.316 - Application Upgrades : 07.02.00 Error!
00:00:42.484 - Application Upgrades : 07.02.01 Success
00:00:42.549 - Application Upgrades : 07.02.02 Error!
00:00:42.598 - Application Upgrades : 07.03.00 Success
Error details as below:
AssemblyVersion7.1.2PortalID-1PortalNameUserID-1UserNameActiveTabID-1ActiveTabNameRawURL/dnndev/Install/UpgradeWizard.aspx/RunUpgradeAbsoluteURL/dnndev/Install/UpgradeWizard.aspx/RunUpgradeAbsoluteURLReferrerhttp://localhost/dnndev/I...l/UpgradeWizard.aspxUserAgentMozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like GeckoDefaultDataProviderDotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProviderExceptionGUID748c3cc4-a2fe-4e28-9e25-40dba3d103c5InnerExceptionVersion string portion was too short or too long.FileNameFileLineNumber0FileColumnNumber0MethodSystem.VersionResult.SetFailureStackTraceMessageSystem.ArgumentException: Version string portion was too short or too long.
at System.Version.VersionResult.SetFailure(ParseFailureKind failure, String argument)
at System.Version.TryParseVersion(String version, VersionResult& result)
at System.Version.Parse(String input)
at System.Version..ctor(String version)
at DotNetNuke.Common.Utilities.CBO.HydrateObject(Object hydratedObject, IDataReader dr)
at DotNetNuke.Common.Utilities.CBO.FillObjectFromReader(Object objObject, IDataReader dr)
at DotNetNuke.Common.Utilities.CBO.CreateObjectFromReader(Type objType, IDataReader dr, Boolean closeReader)
at DotNetNuke.Common.Utilities.CBO.FillListFromReader[TItem](IDataReader dr, IList`1 objList, Boolean closeReader)
at DotNetNuke.Common.Utilities.CBO.FillCollection[TItem](IDataReader dr)
at DotNetNuke.Services.Installer.Packages.PackageController.GetPackagesCallback(CacheItemArgs cacheItemArgs)
at DotNetNuke.Common.Utilities.DataCache.GetCachedDataFromRuntimeCache(CacheItemArgs cacheItemArgs, CacheItemExpiredCallback cacheItemExpired)Source

I have stucked here. could you please suggest how to resolve this.