You should be able to replace the 2600 with the 2600k, however you might need to update your bios first.
easy is a relative term. to me it is easy. you'd have to remove the heatsink, take out the old cpu, put the new cpu in, probably clean the old thermal grease off of the heatsink, apply new TIM (Thermal Interface Material) to the cpu, and then put the heatsink back on.
The 2600k is not faster than the 2600. The "k" means it has an unlocked multiplier which means that it can be overclocked alot further than the 2600. Basically it'll be faster than the 2600 if both are overclocked to their max. If you are going to (heavily) overclock, you'll need an aftermarket heatsink.
if you don't plan on overclocking, there'd be no point in getting the 2600k over the 2600.