Hello, and welcome.
I can give you instructions to replace the batteries, if you don't have them or have never done this before, but, as far as the combination is concerned, if your father forgot it, the only way to retrieve it is from the manufacturer, Sentry. Codes are not available online, as that would compromise the security of your safe. However, this particular model comes with two sets of keys: the four-bitted key to use in conjunction with the combination AND an 'overrirde' or skeleton key to use to open, in just your situation. To access the override key, follow these directions:
If your electronic lock fails or you do not have any codes
available, you may use a key override to access this unit.
Remove the battery cover, disconnect and remove the
battery holder. (there should be a plastic panel under your keypad; look for a small hole towards the left or middle and take a penpoint or the end of a paperclip, push it into the hole, and slide the panel off to the right; inside, you will see the battery holder.)
Insert the skeleton key (second set of keys provided)
into the key hole inside the opening for the battery. (behind the battery holder)
Turn the key to the right.
Turn the four-bitted key to the right.
Open the door
If this doesn't work for you, please visit this webpage and follow all directions, to retrieve the code and you will access the battery holder and replace the batteries, as indicated above (use 4 AA alkaline or lithium for electronics batteries, but never non-alkaline or rechargeable):
You will need to prove safe ownership with a Notary Form, which is a link you can click on at the site, above. If necessary, you an also request expedited service for an additional fee, and have the combination in 48 hours (business days, only).
I hope you can use the override key and get the safe open; however, you will still need the correct code, or you can continue using the override key to open the safe, if you wish.
Please be so kind as to rate my answer with positive feedback; that is the only way I receive credit for my assistance. Thank you very much!Providing a positive rating will not end our conversation, should you need me for any follow-up. Simply click 'Reply' for clarification or additional information, if needed, and I will be glad to continue.Warmest regards,Cher