Get the full package and try installing it. If that doesn't work, repair install; else, uninstall and install again. Make sure to get updates from Windows Update after installing.
The page Install the .NET Framework 3.5 on Windows 11, Windows 10, Windows 8.1, and Windows 8 (code) proposes to install .NET Framework 3.5 via offline installer of .NET Framework 3.5 SP1. This ...