Briefcases usually come with an obvious combination built in--one that any thief could guess. To make up for this security flaw, briefcase locks are designed to be very easy to to reset. That way, the owner of the briefcase can come up with a combination that he can remember but (hopefully) no one else can guess.
Turn the lock to the correct combination and open the briefcase.
Look at the lock. There should be one or two pins that you can slide with your fingers. If there is one, simply push it in. If there are two, push them together. It may be the same button you use to open the case.
Change the lock to an easy-to-remember value, such as 000 or 123, to test the lock. Release the pin.
Close the briefcase and turn the lock to another value. It should not open. Now turn it to the test combination. It should open right up.
Repeat Steps 2 and 3, but this time choose a number that will be easy for you to remember, but hard for someone else to guess. Write down your new combination and keep it somewhere safe.