How do I stop using thumbprint and use passcode

To stop using TouchID - therefore forcing use of the device Passcode - you simply delete all of the registered fingerprints from the device:

Settings > TouchID & Passcode > [Fingerprints] - for each fingerprint, touch the entry, then select Delete Fingerprint

Once all fingerprints have been deleted, your iPad can only prompt for your device Passcode.

Firstly, go to:

Settings > Touch ID & Passcode > Use Touch ID for

Then flick all the switches to disable the feature.

Hope this helps.