If you need to change the email address used to log in to your Forge account, you can do this by adding the new email address as a user and then assigning it administrator access.
Steps
Log in to your Forge account.
Open Settings.
Go to the Users section.
Click Invite User.
Enter the new email address you want to use for the account.
Send the invitation.
Ask the new user to accept the invitation and complete their account setup.
Once the new user has been added, open their user settings.
Change their role to Admin.
The new email address will then have administrator access to the Forge account.
Important
The login email is associated with the user account rather than being directly edited on the existing user. Therefore, instead of changing the email on the current user, you should invite the new email as a separate user and make it an Admin.