Bring it to an authorized repair shop or contact HTC and ship it in. Make sure that it is actually a hardware problem, and then have them replace it.
Try not to replace it yourself, there is a ton of adhesive and you might damage the phone as you need to use a suction device to remove the screen.
(I do believe the buttons are part of the lcd/digitizer assembly, but you might want to check with HTC support)
If you want to save money, https://play.google.com/store/apps/d...yenabler&hl=en
will get you on screen buttons