How can I get rid of old Apple ID on IPad

If your device is Activation Locked, this help page will provide most of the information that you’ll need:

https://support.apple.com/en-us/HT201365

If you have forgotten your AppleID or associated password, these can be recovered here:

https://iforgot.apple.com

To recover your credentials, you’ll need access to one of the following:

• Your primary email address mailbox that corresponds with your AppleID
• Any of the secondary/recovery email addresses that should be configured for your AppleID account
• Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID

Unless you AppleID account has not been fully/correctly configured, or has been seriously neglected, recovery of the AppleID and password should not be difficult.

More information about recovery of your AppleID:

https://support.apple.com/en-us/HT201354

More information about recovery of your AppleID password:

https://support.apple.com/en-us/HT201487

When you recover access to your credentials, you would be well advised to log-in to your AppleID account from a web browser - and verify/update any email addresses and trusted telephone numbers:

https://appleid.apple.com

A window where? What are you doing when it appears?

Normally a request for an Apple Id out of nowhere implies an installed App that is trying to be updated was obtained with that Apple ID, and as such is still requiring it.

You would need to delete the App that is tied to that Apple ID to stop the popups.

Outside of that, make sure you are correctly signed in to your current Apple ID in Settings->Account(top of Settings), Settings->iTunes & App Stores->Apple ID, Settings->Messages->Send & Recieve->Apple ID and Settings->Facetime->Apple ID.