Q: how can I remove a device from my account

Go to https://supportprofile.apple.com and log in with the Apple ID that device is currently registered, click on the device and in the device details page click on "Remove Product"

What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

What to do before selling or giving away your Mac - Apple Support (it says Mac but also applies to iPads and iPhones)

Hi

Go to settings find my ipad top left of page devices remove the device you not longer want.

Then restore device to factory settings then it ready for new user .

Cheers

Brian