.NET 3.5 and modern .NET (Core/5+) run completely independently. You can have all versions installed.
: You must have a USB or DVD containing the exact version of Windows 10 currently installed on your system. Net Framework 3.5 Offline Installer Windows 10 64 Bit
This is where the offline installer—specifically utilizing the Windows 10 installation media (ISO) or the dedicated Developer Pack—becomes invaluable. The offline method bypasses the unreliability of internet downloads by utilizing the local file system as the source. The most efficient technique involves using the Deployment Image Servicing and Management (DISM) command-line tool. By pointing the DISM command to the sources\sxs folder found on a Windows 10 ISO file, the user instructs the operating system to install the framework binaries directly from the disk. By pointing the DISM command to the sources\sxs
Microsoft does not provide a standalone executable (an .exe file) for .NET Framework 3.5 that functions as a traditional offline installer for Windows 10. Users downloading a "dotnetfx35.exe" from the Microsoft Download Center will find that this executable is designed for older operating systems. If run on Windows 10, it typically fails or redirects the user to the Windows Update mechanism. If run on Windows 10