How do I correct the email address on my ipad for icloud

My iPad has my old email address setup for my Apple ID with iCloud and I don't know the password for that email address. I have added the correct contact information in the Apple ID section under iCloud on the tablet and online inside iCloud, but it still shows the old email address in my ID on the tablet. Please instruct me onhow to change the email address, it keeps asking for that password and I have no way to rember that, it is an OLD tablet. Thank you.