Found the warranty. It says I've to try replacing the battery before I return the calculator. The warranty also says it become invalid in case of improper service :s
And there's nothing about replacing the battery in the manual (it just says rechargeable AAA batteries is strictly forbidden and other useless stuff..)
I found a video about how to replug battery on TIs site link
(Now I know you should unplug the white thingy from the calculator and not the cords from the battery, heh..)
But I don't find any videos or tips about how to unplug it. Help?