The "upgrade with bones" feature don't work. Or maybe I just don't understand as usual ¯\_(ツ)_/¯
When you play the game, you can collect bones and see the number of bones you've collected at the upper left corner of the screen. If you have more than one bone, you can activate a power-up by pressing the 'F' key, which will last for a few seconds.
The power-up allows you to kill zombies with just one shot, making it easier to progress in the game. The more bones you collect, the longer the power-up will last when you activate it, giving you a greater advantage against the zombies. So, try to collect as many bones as possible to make the most of the power-up!
I should include this in the description, my bad :D