Your safe's combination is randomly selected and is specific to your safe. Sentry recommends storing your owner's manual in a safe place and not in your safe, as it lists both your safe's combination and serial number.
You will need to provide Sentry with your safe's serial number in the event you lose or forget your safe's preset combination. The only way to retrieve the preset combination is to mail a notarized letter with your safe's serial number and a $6 money order (as of 2011) to Sentry.
Sentry Group
882 Linden Avenue
Rochester, NY 14625-2784
You can change the combination on your Sentry D880 safe, but according to Sentry, it must be done by a professional locksmith as it requires specialized tools. If you change the preset combination to a personal one and forget the new combination, Sentry cannot help you open the safe, as the new combination is not kept on file.