Alfred, I totally agree with what Krikor Boghossian says, you should backup everything,and then - only then, do any changes.
There's a post suggesting a way of Joomla update, which (I suppose) should work for K2 as well, however I am not sure about this, you should check this for yourself:
www.squidoo.com/how-to-upgrade-joomla-website-new-flash-speed-method
Please, let us know how it works out.