On Windows 10 or Server 2012 .NET Framework 4.5 is preinstalled.
If you have an Application which needs an older .NET Framework such as .NET 3.5 you can’t just install it from Windows Features. This is the easiest way to install it:

1.) Download Windows 10 ISO File http://winfuture.de/downloadvorschalt,3226.html  

2.) Mount your ISO File in Windows Explorer

3.) Make Sure the ISO File is mounted It should look like this. Note your Driveletter, in our case D:

4.) Open an elevated Command Prompt

5.) Type or Paste this Code in and press Enter. Make sure the Driveletter is right.

Dism /online /enable-feature /featurename:NetFX3 /All /Source:D:\sources\sxs /LimitAccess

6.) After this we have successfully installed .NET 3.5