I had an issue with icons not displaying. It was due to moving to a newer version of IIS. A module installed Static Content mime types .SVG .WOFF and .WOFF2
.Woff2 is already added in newer versions of IIS. Having this in the web config caused icons not to show up on my site and is a known IIS issue if you declare it twice it will bug out your icons.
open your web.config file and see if staticContent is present and remove the mimeMap for .Woff2
To answer your question about an upgrade, after making a complete backup of files and database... you can upgrade just by uploading the upgrade package then type this URL in
you should be able to walk through the upgrade process.
I hope this clears it up for you if I am reading this right.