Replacing the rear rotor on a K bike is easy. All you need to do is take the rear wheel off and then remove the two bolts that hold the rotor to the final drive.
BUT, and it's a very big but, oftentimes, due to corrosion and the Loctite used at the factory, the heads of those bolts can strip even if you've preheated them with a torch to burn off the Loctite. In that case what you'll need to do is drill the heads off of the bolts, remove the rotor and then extract what's left with some Vice-Grips.
Here's an example where one bolt came out but the head on the other one stripped. I drilled the head off with a 5/16" drill bit:
Then there will be enough of a bolt left to unscrew with some Vice-Grips. Make sure that you get the Vice-Grips as tightly on the bolt as possible. Otherwise when you turn the Vice-Grips you'll just strip metal off of the bolt. If on the bike then have the bike in gear when removing what's left of the bolt. On the bench you can put in two of the lug bolts in and use a pipe to get some leverage and keep the final drive from turning:
The bolts can be replaced with countersunk M8x20 bolts from the hardware store.