Click on the license number in the user's or the owner's account to bring up the Modify License window.  Click the Transfer button at the top of the window.  Enter the new User's email address.  If the Owner is different than the User (for example, an agent owns a complimentary license issued to a user), make sure to enter the Owner's email address as well.  If you don't enter an Owner's email address, the User will become the Owner of the license as well.