If you want to change your PayPal name (a "username" that is often called in common terms), you can do it by logging into the PayPal website (not the app), going to Settings, clicking "Change name, " and following the prompts, but you can only change it once for small errors, and if you want to do a major legal change (like changing your name after marriage), you will have to show your ID. In case you are referring to changing your login email/username, the update is done in Settings under Contact Info; if you have major changes or issues, you might need to get in touch with support directly.