You will need to get assistance from Mendix Support by writing an email at support@mendix.com

In the past it was possible to change the email address of your Mendix account via the portal.

For unknown reason Mendix does no longer allow to change your email address and they force you to create a complete new mendix account. Then they say they can merge your accounts, but that does not apply to your projects.

It's bad design of the Mendix platform where they want the user name to be the same as the email address.

A beginners mistake on multiple places on the platform is that Mendix sends emails to your Mendix user name instead of to your Mendix email address :-(

Mendix support is not able to change your email address either.