Currently there is no way to change your email address for your Birda account, but this is something our team is working on and we hope to bring this to the app soon.
Maybe you've logged in to find you've lost all your sightings? Here's a support article to help with this - https://support.birda.org/en/articles/6297152-lost-all-your-load-sightings-and-sessions-after-logging-into-birda