Edit your linked email
You can only change the email linked to Step if you had originally created the account with an email and password. To change it, go to Profile > Settings > select 'Change email' from the list.
If you had originally created your Step account by signing in with Google or Apple, you will not see the option to change your email because your Step account is associated with your Google or Apple profile and you must always sign in by pressing the "Sign in with Google or Apple" button.
Edit your username
From the Profile tap on the pencil icon > edit the username.
The username must be between 6 and 16 characters, and cannot include capital letters or special characters.