My HTC one droped from the stairs how/where can I fix it

There is definitely some severe damage to the phones exterior. Like the back casing, display, and the front housing the display sits in. These parts can always be replaced so that is not an issue. I am curious if the drop caused any significant damage to the phones interior parts such as the motherboard, daughterboard and cables. Depending if you are wanting the phone repaired to save the current data you have saved to the phone or if you aren't concerned with the data and just like the HTC One then this could help determine if it is worth the cost to repair it. You may be able to find a used HTC One in good condition for less then what it would cost to repair the current phone. If you are wanting to repair your current phone then you may want to remove the back casing from the phone and look for any obvious damage such as cracks in the logic boards, broken off or missing components as well as broken or torn cables and so on. You can you the guides on this site for some guidance. If the interiors of the phone seem undamaged and intact I would then see if the phone can still power on. If you push in on the little button where the power button should be and soon after doing this you hear or feel the vibrator motor run then try plugging the phone into a PC to see if the PC can recognize the phone. If so then the phone is very repairable and your data is retrievable. If you do remove the back casing and suspect there may be damage you could post more pictures to have some one confirm the damage and if it is repairable. So what I am getting at is try to make sure the interior parts of the phone are functional before buying a new back case and front housing frame with the display. I do not know where you are located but if this is not something you feel comfortable doing yourself then have a professional look at it. I, as well as others on this site offer mail in repairs so you could always browse through some of the user/members pages to find someone. I hope I was helpful and let me know if you have more questions. Best of luck.