removing my son's username and account and using my account name on my mac.

I let I son use my MacBook a few months back before I had iTunes music. I just recently signed up on my phone and wanted to listed to my music on my Mac. When I go into account it has his email information instead of mine. When I enter my username and password it flashes back to his email for the Apple ID. How do I remove his email and use mine to access my apple music on my Mac.