Yes you can! Here's how:
First, open your account settings and add the new email address as an Alternate email address to your Fleep account.
Once you have confirmed the Alternate email address, you will be able to set it as the new primary email address by hovering over the address and clicking on the 'Make primary' button that appears:
More info about your primary email: your Primary email is the email address you used when you signed up for your Fleep account. When you used Google sign-in or Microsoft sign in the first time to log in to Fleep, then your Gmail or Microsoft email account is the Primary email address.