You can change the email address you use to sign in by logging in with your current email address and navigating to the user page from the menu at the top. Click on your name and email address to proceed.

On the user page, you can update your email address by selecting "Update Email Address."

Once updated, you can sign in with your new email address.

Would you also like to change the email address used to send emails to your customers?

If so, go to Settings > Email Settings.