Happy to assist you in locating the battery for your safe. I do not see in your question about what safe you own but questions on our site often refer to the Sentry brand. In general, start by looking for the bright yellow tag that is located inside the safe door. The battery compartment is located there. Use a screwdriver to open the compartment for the battery. You can then pull on this compartment and replace with new 4 AAA batteries.
I would also be happy to locate the manual for you for this particular safe to assist you as well. Please let me know the safe model so that I can research it for you.