Windows 10 comes with .NET framework 4.5 pre-installed, but many apps developed in Vista and Windows 7 era require the .NET framework v3.5 installed along with 4.5.

These apps will not run unless you will install the required version. When you try to run any such app, Windows 10 will prompt you to download and install .NET framework 3.5 from the Internet. However, this will take a lot of time.
You can save your time and install .NET Framework 3.5 from the Windows 10 installation media.
This method is much faster and does not even require an Internet connection. Here is how to install it.

To install .NET Framework 3.5 in Windows 10, do the following:

1. Insert your Windows 10 DVD, or insert your flash drive with Windows 10 setup, depending on what you have.

2. Open 'This PC' in File Explorer and note the drive letter of the installation media you have inserted. In my case it is disk D:

3. Now open an elevated command prompt (Run Command Prompt As Administrator) and type the following command:

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


Replace D: with your drive letter for Windows 10 installation media, WHERE D: is the letter of your flash drive, Windows DVD or hard disk you inserted

You are done! This will install .NET framework 3.5 in Windows 10.