Choose a combination. Pick a set of numbers that is relevant to you, that you won't forget, but that isn't too easy for someone else to guess.
Open your briefcase. If your briefcase is new, there should be a factory default combination given to you with your product information.
Locate the reset button. This will likely be located on the backside of the lock or very near by. It will look like a slightly inset button.
Reset the combination. Use a ballpoint pen(or other small pointy object) to depress the reset button. While you're holding down the button, move the combination wheels to show the code you chose. Release the reset button when you're done.
Check to make sure the reset worked. Shut and lock the briefcase. Spin the combination to a random set of numbers and wait a few seconds. Return the combination to the code you just set and make sure it opens. If it doesn't, try the reset process again. Make sure that the reset button is pressed firmly the whole time.