Hello, I have two htc One A9s and the batterys are nearly dead all the time. I wanna know, what replacement battery is the best, and how I should do the replacement, because i never opened a phone the "normal" way before. Are there some things, that could easly break and i should have for the repair, or is it only a replacement battery, but from wich source?
I found what looks like a fairly decent video on battery replacement on your phone. It goes step by step and gives you appropriate warnings when needed and also lists the tools used to do the repair. A T5 Torx driver looks like the most unique item you'll need.
Mostly I'd say work slowly and carefully while removing the screen. If you don't have a hot air gun, generally a hair dryer will work fine, or you can splurge a few bucks and buy iFixit's iOpener that works quite well.
In addition, you'll need some adhesive to resecure the screen in place when you're done; precut is definitely preferred. I always pick up at least a couple when I buy them; sometimes it's easy to get them mispositioned and when that happens you don't want to have to wait until you reorder it. There are also times when you might forget something and have to open the phone back up again; I've done that a few times myself.
Here's an example of the adhesive; if you look around at places like Amazon, AliExpress and eBay you'll find plenty more.
Sorry I didn't address this earlier, but as far as batteries go, I've generally had good luck with aftermarket batteries myself. To be safe, look for "Genuine HTC" batteries; those will be what came with the phone originally, but personally I generally try to find a good price on places like eBay, Amazon and AliExpress. Oh, and don't bother with those "gold" or "high capacity" batteries that promise hugely inflated mAh numbers; those have been pretty thoroughly debunked right here on iFixit. They're fine if you want to pay a premium price for an average battery, but otherwise skip them.
@Marten You're in good shape with regard to replacing your screen; it's old enough that the prices have dropped significantly. The first one I came across was only about $11 USD,