How do you change the code for a Sears keyless entry garrage door opener if you do not know the code that it is presently set at?

First you need the Make and model(Lift Master, Chamberlain, etc.)
Some are DIP, most are "learned"

Model number would help. You can buy or download the owners manual for most and then follow the instructions. Mine is a learned type system, you don't have to know the old code and there are ways to wipe the codes completely from the system.