I found this quick video on youtube, where you can see how to reprogram a chamberlain Garage Door Clicker and how to change the battery. I hope you wont use 1234 as your access code. Enjoy video.
How to Reprogram a Chamberlain Garage Door Keypad
January 18th, 2010 
Tom Bricket 
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Five stars I love that keypad very nice mine is a liftmaster and its newer but its the same company.
my friend. After hours of searching on the web, you have given me the only advice that was easy, yet instructive. Your instructions work for liftmaster too. Thank you. 5 stars.
I don’t know, but thank you for giving it five stars.
Who rated this video 1 star? I gave it 5 stars!
Thanks buddy! Moved into this house and didn’t have the code. Thanks again.