- C22-UX31 ASUS battery or a generic equivalent. (I got one off Amazon Germany for about 70€)
- T5 Torx screwdriver
- small Phillips screwdriver (#00, I believe)
- small spudger, flat screwdriver or other prying tool
Remove the 10 Torx screws holding the bottom cover. The two at the back edge of the device are longer (blue circles in the image on top). Take out the 3 Phillips screws holding the battery in place (red circles).
The battery's board connector is a bit tricky, as it's snapped into its plastic jack by the two small knobs marked by red arrows in the image below without any lever mechanism. Carefully pry the jack open with a small flat tool and remove the battery. Do everything in reverse with the new battery, charge for a few hours and enjoy.
Notes: As the Zenbook is flat enough to be used as a meat cleaver, the interior is extremely compact and well organised. Also, that tiny bar under the SanDisk sticker is the SSD. Should have guessed, but was still shocked (Also, it's more or less proprietary. Bad news if it ever breaks down).
We took the opportunity to clean out the fan with canned air, but there was hardly any dust even after 18 months of sloppy use. Probably due to the fact that the laptop gets a lot of passive cooling via the metal body and every opening is mesh covered from the inside.
Not so impressed by the original battery performance. I still get about an hour of use out of it, but it tended towards shutting the system down without warning at about 50% nominal charge.