We need to change the Administrator Information on this Mac.

Good Morning,

We got new Mac's for our office about 2-3 yrs ago and just let everyone set up their own trusting that they would set the administrator to Woods.Electric and using a password that we keep in our files. About 3 Months ago the employee that set this Mac up "Alexia's iMac" is no longer here and now we can't do anything because she didn't use the credentials she was told to use when setting up and she didn't leave any clues as to what she set the admin name & password to. We have years of documents, data, and other important information saved on this computer so I really don't want to have to factory reset and lose everything. How can I change the Administrator info with out resetting my computer. Since Woods Electric purchased these computers and it's our appleId that makes any purchases or changes there has to be away to do this with out loosing everything we have.