Easy to replace. The valve fits into a slot on the back of the toilet. When you order the valve you get a neoprene donut used when bolting the toilet to the floor. My toilet was mounted too close to the wall for the valve to completely slide out, so i had to unbolt the toilet from the flange to finish removing the valve. I needed the donut when remounting the toilet.
To remove the valve reqiures disconnecting the supply line and the tubing to the toilet rim, removing one screw and disconnecting the flush lever rod.
This video is not mine but shows the installation of the new valve.
Video