Step 1:

Click Start. Go to Settings. Choose Control Panel then choose Programs.

Step 2 :

Click Turn Windows features on or off, and the user will see window as image below.

Select .NET Framework 3.5 (include .NET 2.0 and 3.0) select it and click OK. After this step, it will download the entire package from internet and install the .NET Framework 3.5 feature.