Every ProtonMail account has a username associated with it. This username is used as the main email address for that ProtonMail account (firstname.lastname@example.org). Unfortunately, we cannot change the main login username on your account.
When you created your account, you also created a key that was matched to your username. This key has allowed you to send and receive emails in the most secure manner. By changing your username, you would also be changing this key which has been used to send and receive all the emails in your inbox.
You are welcome to create a new account at protonmail.com/signup.
You can also add additional usernames to your account. For more information, see Aliases and Addresses in ProtonMail.
If you would like to merge one account into another account, please see Combining accounts.