I don't have any experience with changing the battery on the MC200 specifically but have changed them on several other PCs
The MC200 is basically a headless PC and, apart from the large number of screws, it has good access so it should be quite straight forward - I have in the past changed HDD and memory modules and takes under 15 minutes.
My only concern would be what state it will be in after battery is disconnected as there is no keyboard/display/beep to signify a bios reconfiguration. But I probably would have gone through same process when I changed my memory cards - later removed as they made no difference and one module was faulty.
To get access, iirc you need a small Torx driver, remove the feet then with a little force, try and slide one of the sides off. Iirc that both will slide off so, for the inexperienced, its pot luck whether you get the right one first time, hence my estimate of 15 minutes for the job. Its a smooth action to slide but does require a bit of a push. If it doesn't move then try to slide in different direction but off top of my head, they slide towards the bottom hence why you need to remove the feet.
Once side is off, there are then many Torx screws to remove, then you will see either the top side or bottom side of motherboard. I think you can actually peek into the case and see a few internal components to give a clue to what side is what.
Reassembly is simply the reverse. Pretty straight forward.