When you create a new account in the blast app the first step is creating a username (email address) and password. The username is the main link to your account and metrics. The username cannot be changed.
If you need to change email address, you have to start a new account with a different email. This is necessary because the old account will still be in the system. Since the metrics don’t transfer, you would still have access to the information by logging into the original account.