If it's a combo lock this should work (worked for me on a combo guitar case I "inherited".)
Pull the sliding side latch to the side, as you would if you were opening the briefcase normally. Holding the latch in position, lightly roll your fingers down over each numbered wheel to identify which one is the tightest. The tightest wheel will give the least when you gently tug on it with your fingertip.
Set the tightest wheel to "0." Check the tightness of the other two wheels; if they still feel loose, click the zeroed lock to "1" and feel the other two wheels again. Keep going in this fashion until one of the other two wheels feels tight. When one of the other wheels becomes tight, the first wheel is set at the correct number.
Move on to the second tightened wheel, continuing to hold the side latch in place. Set it at "0" and check the final loose wheel. Keep adjusting the second wheel, as you did with the first, until the third wheel feels tight.
Set the final wheel at "0" and adjust it until the briefcase's top latches pop open. Make sure the sliding side latch is still being held in place.
Reset the combination to numbers you'll remember in the future. Write them down and store them in a safe place, just in case.