We recently purchased a home with an existing garage door opener (craftsman). There's a keypad outside but there is no manual so we don't know how to change the code. Any suggestions?

You did not give the model number of the garage door opener.

Over the years there Chamberlain has used several different systems.

Here are some generic programing infomration.

http://www.chamberlain-diy.com/diy04/faq_view/0,1341,uid-d2t7cGFkZHJjZA==_1449-id-9,00.html

Chamberlain makes the Chamberlain, Liftmaster and Sears Openers and they all use similar parts and proeedures.