If you want to upgrade from .NET Framework 4.7.2 to .NET Framework 4.8, you can if you are running CU2 for Exchange 2019 or higher. You can look at the Microsoft Support Matrix to see what is supported but here is a snippet from the Microsoft Site:
Download the offline installer, it is around the 111mb mark.
Now you can run the installer, .NET has a whole new looking website where you can get different flavours of .NET to download.
The extraction and preparation starts.
Accept the terms and click Install.
Progress screen of the installation..
There you have it, successfully installed .NET Framework on Exchange Server 2019 CU3 and Exchange 2016 CU14.
Hope it helps.