Find a locksmith or licensed Gardall distributor in your area. You can find one by going to the Gardall website and entering your ZIP Code in the "find a dealer" box on the left side of the home page. Another option is to call the security business that sold you the safe, though they're likely to be listed on Gardall's website anyway.
Make an appointment with the dealer or locksmith to provide you with a combination key. This will allow you to access your safe and change the combination. Pay for the key and be sure to get instructions from the dealer or locksmith.
Follow the instructions provided by the dealer or locksmith to change the combination on your safe. Follow Gardall's "Mechanical Lock Dialing Instructions" to open the safe with your new combination.