How can I open the backside for battery replacement?

Hi everybody,

Due to the incredibly bad battery performance of my HTC One A9s, I decided not to buy a new phone but to replace the battery. However, I bought the battery and a little stick/tool to open the backside, but it does not open without any potential damage. Since it has a clear opening and something like a groove in its design, it shouldn’t be that difficult, right? There appears to be no information or youtube-vido whatsoever on this topic on the internet, so can anybody help me getting it done?

Thank you a lot in advance,

Kamil