Note that for this to work the new email address you plan to use cannot already have an "AdobeID", it needs to be an email address that is new to Adobe... If you are unsure, you can try to sign into any Adobe service and click "Forgot My Password", it will let you know if there is an AdobeID associated with that email address or not.
When you are signed into FormsCentral click on the "Account" icon (looks like a person at the top right):
Select "My Information":
In the "My Information" dialog you can change your AdobeID email address:
After changing the Email address in this dialog you will have to "Verify" your email address, Adobe will send an email to the address with a link you need to click or copy/paste into a web browser to Verify the Email.