The changes couldn't be completed.Please reboot your computer and try again.

https://social.msdn.microsoft.com/Forums/en-US/4ea808e7-c503-4f99-9480-aa8e6938be3d/net-framework-35-doesnt-install-windows-10-error-code-0x800f081f?forum=netfxsetup

 

I had this problem as well on my machine at work.
Try doing the following it worked and allowed me to download the .Net Framework 3.5 on my Windows 10 PC : 
Go into RegEdit and set the HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU folder. 
DoubleClick on UseWUServer(UseWindowsUpdateServer) and change the value to 0. 
If the value is already 0 this fix may not work for you. 
You may need to restart afterwards.