You need to change the attribute of the file so the read only is turned off. Open Notepad (run as Administrator). Using the Open command on Notepads menu, navigate to the file you want to edit. Rt click on the file and choose Properties. In the General tab, under Attributes, untick the Read-only box and click OK.
Now open in Notepad, edit and save. Remember to go back to the properties and make it Read-only again.