When you sign up on KissKissBankBank, you chose an email address to your account.
You can replace this email address with a different one only if this second address has not already been used to create another KissKissBankBank account.
To do that, sign in to KissKissBankBank, click on your profile picture or username at the top-right corner of the page and click on "Update my profile".
At the bottom of the page, you can change your email address. Don't forget to click on "Update my profile" to save the change.