1
Go to the menu at the top of the Fritz program, and choose Tools > Book options. This can also be done by simply hitting the F4 key, which will bring up the opening book options box without having to go to the Tools menu.
2
Click on the "Optimize" button for maximum learning potential and least variety of play, making it play the strongest moves it knows in a given situation. Click "Normal" for factory settings, which will allow some variety of play and decrease the learning potential, which is best for playing against it as a learning tool. Click the "Handicap" button to activate totally random opening moves, and zero learning potential, weakening the engine substantially.
3
Change the "Minimum games" and "Up to move" boxes to reflect how many games it takes before the engine permanently changes a move order and how many moves into a chess game the engine will still consider itself in the opening stages.
4
Click "OK" once all the settings are to your satisfaction, and Fritz will automatically load them every time it starts up, until they are once again changed.
Tips & Warnings
You can also left click the sliders in the book options box to make totally custom book learning options.