How do I switch to my original Apple ID on a new MacBook Air?

Regret doing a new Apple ID, need help to restart from the beginning to use the one I’ve always been using.

Got a macbook air, and instead of using the apple id I always use for the rest of my other devices i made a new one. Regret doing that cause I’m starting college and might need the data from the id i always use. How do i go to the start to use the id I've always been using?

[Re-Titled by Moderator]