I had the same problem following a fast reverse into a fir tree - fortunately it was pretty low so the only damage was some grass stains and the batter cover coming off.
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To repair I had to first remove the 2 rear screws from the underside of the Mavic mini
Then pull the top casing up slightly = note that this tears the sticker on the right hand side - so I'm guessing that means any warranty is now lost.
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After that I managed to get one side of the battery cover to clip in
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To do the other side I inserted a couple of bits of plastic ties just under the top case...
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And kind of wiggled the second lug from the battery cover roughly into position and then pushed and twisted it using a small screwdriver.
This shot was taken just before I refitted the screws and you can see the top is still slightly lifted on the right hand side.
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Popped the rear screws back in and tightened up and it's back flying again...