Professional on closed course, do not attempt this at home
The machined aluminum unibody and zero gap construction makes the HTC One a very attractive phone. It also makes it extremely difficult to take apart. Scoring a 1 out of 10 for reparability, iFixit warns that it is nearly impossible to open the phone without damaging the back case. Dead batteries and shattered screens are going to be problematic, since iFixit found them nearly impossible to replace. Make sure to keep that warranty number and accidental insurance handy, because like a fine European car this phone is not cheap to repair. On the plus side, the solid construction makes for great durability.
Bottom line: make a case the first accessory you buy for the One.