I think the way it works is --- remove the battery (or whatever powers it) from the keypad. Replace the battery after a minute or two - the keypad should now be cleared. Then you have to push a button somewhere on the opener itself - probably on the back, there will be a pilot light and a switch next to it. (When you push the button, the light will blink.) Then, push "Learn" (or some such key) on the keypad so that the opener will recognize the keypad's signal. You may have to push that button for 5 seconds. Then enter the new code (4 digits) on the keypad. Wait 30 seconds for it to register/save.
I think that's it if memory serves. You could try asking at Sears - they can either pull the instructions from a new door opener or give you the 800 number to order a new set of instructions ($5.00).