How do I change stored username

Open Settings and find Passwords in the list. Find the username / password you want to change in the list, press in and press Edit in the upper right corner. Now you have the opportunity to change your username.

REMEMBER! Changing the username inside your iCloud Keychain does NOT change the username you have on the associated website. It requires that you change the username of directly with that provider. Changing the username in Keychain is just a help, so you do not have to enter your username and password when you need to log in.