If you are using Sign in with Apple for your Kickstarter account, it is not possible to change your email address or your password directly through Kickstarter.
You can manage your Apple login credentials via your Apple ID settings on your mobile device, or from your Apple account. This is also where you can manage the apps on which you use Sign in with Apple. You will be able to turn off email forwarding (if you are using the relay email address), change your forwarding address, or stop using your Apple ID with an app too.