If your username is a word, such as johnsmithpgeaccount, it cannot be changed. If your username is an email address (such as "firstname.lastname@example.org") then it can be updated in Profile & Alerts.
To change your username, sign in to Your Account using your current username and password. Select EDIT PROFILE & ALERTS (yellow button at the top of the page). This will bring you to the PG&E Preference Center. From there, scroll down to Profile and select Username, Password. Scroll down to Change Username. Input your new username, and then confirm your new username. Select SAVE CHANGES at the bottom of the page when you are finished.