Hello Seek:  

Thank you for your question, also... 

Did you know there are two DNN conferences coming up?  DNN Summit in February, and DNN-Connect in May.  :)

The answer to your question is both easier and more difficult than you might thing, in my opinion.  

Hopefully, you have all of the modules in a single solution.  If you don't you'll just need to repeat these steps in a different instance of Visual Studio, or just repeat them over and over again and testing.  Ideally, you should try to get them all into a single solution, in the long term.  

1. Fork or branch the code according to your corporate policy and chosen source control.  
2. Open the solution for the module(s) in question.  
3. Update the .NET compile target to 4.7.2 or 4.8, depending on your preference.  Both should work fine for now.  
4. Update the related dependencies to compile against the correct (newer) version of DNN you wish to upgrade to*.  Ideally, this will be as easy as updating all of the Nuget references.  Otheriwse, it might be a simple or convoluted local reference that you may need to reverse engineer.  
5. Build the solution.  
6. Resolve any compilation errors (and also any deprecation warnings).  Nearly all of these will be as easy as one line of code difference.  (If you see deprecation warnings about NavigateURL(), I'd suggest skipping that one for the moment.)
7. Repeat steps 5-6 as many times as necessary.
8. Once you have clean builds, test your modules on a new test instance of DNN that's upgraded to or past your target DNN version and test thoroughly there.  
9. Once testing & remediation is successful, build your upgrade plan to commit and schedule deploying everything in production. 

* Example:  If the version you're trying to move to is 09.10.02, in many instance you should target 09.10.00 and not 09.10.02, but you can do either as long as things work.  

I hope this helps!  :)