open up System Preferences from Dock or Launcher
Then open security & Privacy 
Click lock icon on left corner of Security & Privacy Window 
Now it will ask your admin Password to grand access 
“System Preferences is trying to unlock security & Privacy preferences. Type your password to allow this “
type your password in the box and click unlock 
Now you can see a section called “Allow application download from :”

  1. Mac App Store
  2. Mac App Store and identified developers
  3. Anywhere
by default it will be 2. Mac App Store and identified developers 
you have to change it to 3. Anywhere 
now a small drop down window will appear for confirmation 
click “Allow From Anywhere”
now unlock icon again to lock window 
Done 🙂 

Here Video

Done 🙂