Windows 10 .NET 3.5 installation

.Net Framework should be built into Windows 10 already and you can enable it from control panel.

Do the following to enable .Net Framework on your Windows 10

1. Press Windows + R keys on the keyboard.
2. Type “appwiz.cpl” in the “Run” command box and press “ENTER”.
3. In the “Programs and Features” window, click on the link “Turn Windows features on or off”.
4. Check if the “.NET Framework 3.5 (includes .NET 2.0 and 3.0)” option is available in it.
5. If yes, then enable it and then click on “OK”.
6. Follow the on-screen instructions to complete the installation and restart the computer, if prompted

This article explains how (not tested)

http://winaero.com/blog/offline-install-of-net-framework-3-5-in-windows-10-using-dism/

Insert your Windows 10 DVD, or double click its ISO image, or insert your bootable flash drive with Windows 10, depending on what you have.
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:
installation media drive d
Now open an elevated command prompt 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.
dism

To save your time, I have prepared a simple batch file which will save your time and will find the inserted installation media automatically. It looks like this:

@echo off
Title .NET Framework 3.5 Offline Installer
for %%I in (D E F G H I J K L M N O P Q R S T U V W X Y Z) do if exist "%%I:\\sources\install.wim" set setupdrv=%%I
if defined setupdrv (
echo Found drive %setupdrv%
echo Installing .NET Framework 3.5...
Dism /online /enable-feature /featurename:NetFX3 /All /Source:%setupdrv%:\sources\sxs /LimitAccess
echo.
echo .NET Framework 3.5 should be installed
echo.
) else (
echo No installation media found!
echo Insert DVD or USB flash drive and run this file once again. 
echo.
)
pause

There is a installer from techgainer.com called ".NET Framework 3.5 Offline Installer" which helps you to fix this problem. For that you just need a Windows installation DVD!