Open the Android System Recovery menu.

You can find the manual how to do this here.

If your tablet is stuck on the Android robot with exclamation mark picture, press the Volume Up or Down button.

Now, if the cursor is on "reboot system now", press the Volume Down key twice to go to "apply update from external storage", and just once to go to "wipe data...".

The cursor will stay on "apply update..." but the real selection will be "wipe data / factory reset".

Press the Power On button to select it. In the next menu you must choose "Yes - erase all user data".

Use the same technique to choose it. And finally, "reboot system now".