You can sign out of the Apple ID on the iPad and sign in with the one you want to use in the following locations.
Settings ➜ Account (top of Settings) ➜ Scroll to the bottom and tap on Sign Out.
Settings ➜ Account ➜ Media & Purchases
Settings ➜ Messages ➜ Send & Receive ➜ Apple ID
Settings ➜ FaceTime ➜ Caller ID ➜Apple ID
Do note, that the Apple ID continues to function regardless of the validity or existence of the email address used.
Also note, that any purchases made with that Apple ID, will forever be tied to that Apple ID and in the case of Apps, will always require it and its password to update.
If there's something you want to keep from that Apple ID, then its suggested you simply update the email address used with the Apple ID to functional one and keep using that Apple ID with it to access its purchased content. click here ➜ Change the email address you use as your Apple ID - Apple Support
Otherwise, you should sign out of the Apple ID on the iPad, remove it from Find My, and erase the iPad entirely to not have anything left that may remain tied to the old Apple ID, then request the Apple ID be completely deleted if you no longer plan to use it. It's very important you sign out of the iPad entirely before requesting deletion of the Apple ID as otherwise you can render the iPad unusable for ever.
click here ➜ How to delete your Apple ID account - Apple Support