The HTC U12 Plus' durability got tested by JerryRigEverything quite recently, and now, Jerry decided to disassemble HTC's flagship, and find out whether its buttons can be fixed, as we was able to cut them off quite easily during the durability test. It's worth noting that those buttons are not your regular physical keys, HTC decided to take a different approach here, and basically include capacitive keys which look like regular physical buttons.
In any case, if you take a look at the video down below, you will be able to see the whole HTC U12 Plus teardown procedure. This will not only give us insight regarding the phone's buttons, but it will also give you a good idea as to how to disassemble the phone in case you need to replace a component, or something like that. So, the first thing you'll need to do is apply heat on the back of the phone, and use a pry tool to get it to open. In the video, however, that was not necessary, as the phone was already damaged from the durability test, so the source was able to open it without applying heat altogether. At that point, you will need to detach the fingerprint scanner ribbon. There are eight screws holding the top part of phone's internals in place, so you'll need to remove them all before moving on.
After you do that, you'll get access to the phone's cameras, battery, and the motherboard. At that point, you'll need to detach the battery ribbon, and both charging port extension ribbons. From that point on, you can remove the phone's pressure sensors, and the motherboard, along with the phone's rear-facing cameras. There are a number more steps you'll need to take in order to fully disassemble the phone. The bad news is, though, those buttons cannot really be fixed, if they get damaged, as the whole frame of the phone will need to be replaced in order to do so, at least according to the source. The source also says that the HTC U12 Plus is one of the 'least repairable smartphones of 2018 so far'. If you're interested in checking out the whole video, it's embedded down below.