How do I unlock a deceased persons iPad?

Here are the Apple procedures to read and follow for this case:

How to request access to a deceased family member's Apple accounts - Apple Support

Basically, gather up the requested account data, and acquire a court order, and Apple will unlock what it can.

There can or will be encrypted data that Apple will not be able to assist with decrypting; if the passwords are not known. Best case there is figuring out the passcode or password. But Activation Lock can be cleared, allowing the devices to be re-used.

Absent a court order or equivalent, and absent an original purchase receipt for the device and matching ID, this situation is little different from acquiring a lost or stolen iPad or iPhone, Activation Lock is a theft deterrent, and there’s no bypass for that without access to the Apple ID that established the lock.

You can

1. If you have the Apple ID & password or
2. If you have the original iPad receipt

Then you have a pretty tray that looks like an iPad.