You need to hold down all 4 or 5 buttons at once. This will reset your watch and bring up the message "welcome to polar fitness world". Press ok button 1x to start your setup process.

I pressed al the 5 buttons at once, was a little bit tricky, but pressed the center button on the right on a table and used my fingers for the other 4 buttons.
Screen went completely black, al the symbols were shown.
After a while, I had to re-enter the settings.
It's working now :D Only problem is that the hr-sensor isn't working..probably also a dead battery.