Changing administration name and account

Hello,

I took my very first Mac Mini to Apple to be repaired which I now use a spare mini, I noticed that Apple had changed the administration name and account to Apple, how can I change this. I am a bit nervous, when I clicked on advance from Users and groups to see if can change but got too nervous to do any changes, can someone please give me a step by step as I do not understand all the terminology and do not wish to cause any problems.

Thank you

Kind regards

rachealfromva