How do I change my username?

I was wanting it to be the about the same, but I wanted it to be “blondieandtay”, so I can remember my dog, Blondie. I’m trying to change it to that on most of my apps that require usernames, because it’s a good memory of my dog Blondie who died a few months ago. Thank you!

Hey Julia! I just figured out how to do it from my device! So I don’t need help anymore, but thank you!

Or “tayandblondie”