How to delete an old Apple ID from my iPhone

I cant use my icloud+ with Apple on because my old Gmail Apple ID is still on this phone. I guess when I purchased it and I transferred my data it transferred that all the Apple ID at the same time so I’m able to use like Apple Music and arcade but I can’t use my plus so I just need to delete my old Apple ID mi**********@gmail.com. Also, I am suspicious that there’s been fishing or however, you spell it because I’m getting all kinds of weird spam and weird glitches.

[Edited by Moderator]