ipad won't let me in ?

When I switch on the iPad I get a box that says: Sign in to iCloud, Enter the Apple ID password for - then shows an old email address that I no longer use.

There is a box for entering the password then Cancel and OK

Every time I touch on the cancel button the same box just comes back again.

I have a new Apple ID that works for the iPhone and mac but how do I get past this Sign in so I can change the email address then enter the new password to log in to the iPad?

Thanks