I hope I am safe in assuming your mother (or you) tried opening the safe with the key and that did on work.
If so, this YouTube video may prove instructive regarding opening a digital safe:
This YouTube video shows another method:
Note: Assuming the safe can be opened following one of the techniques in the video, a new battery / batteries should be installed before resetting the code. (Also, I understand the batteries should be replaced regularly to avoid the battery / batteries becoming depleted and causing problems in opening the safe.I suggest replacing the battery / batteries once a year - note the date on a calendar you will look at regularly.) Plus, you and your mother should write the new code down and put the information in places where you will remember them. I like to put codes in my address book (a physical address book for people's names, street addresses, and phone numbers - very old school.) For a safe code, I have used the maker name of the safe as the "last name" and my first initial as the "first name". Example: Safewell, J. I have also made the code look like a telephone number. For example if it is a 6-digit code for the safe, I have made the code the first 6 digits, followed by zeros. Example: code is 987654, phone number in my address book: 987-654-0000 (USA phone number style.)