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First, turn over the tablet so the screen is face down (on a soft but firm surface, so as not to scratch or damage the screen.)
Then remove four rubber covers, and undo the four case screws at the corners. The two ends of the tablet case come off.
Then, there are eight more screws, two at each corner.
Then the screen can be pulled away from the device, and the connector removed/unplugged, carefully since it is somewhat fragile, and there is a plastic strip on the connector that can be used to unplug the ribbon connector.
Place the screen to one side.
Where the screen was, you should see the battery and the circuit board of the tablet, like this:
and the yellow oblong is the battery.
Undo the adhesive tape securing the battery in place, figure out where the battery connect electrically to the rest of the tablet, and undo that connection.
Replace the battery with an *exact* replacement, not just by size and voltage but also by the other specifications.
Let me know how it goes, and if you need any further help.