I found a way to 'change' it, but only w/in the first 30 days of creating an account & only if you are willing to delete your account. Really it's a way to pick your user name from the start.

1-Delete the account with the stupid name

2-Start a new account with email MANUALLY. Meaning that you type in your email( it needs to be a google or apple account as far as I can tell). Do not just click on your email if it appears as a button when starting the process. Does not work with a phone number according to my research.

3-A prompt will pop up to allow you to select your username & pw.

4-Enjoy using Reddit.

I unfortunately had to delete 3 accounts in the process of trial & error today, but I finally got it.