Installing a toilet is not too difficult, although there are a fair number of steps. If everything goes smoothly, you should be able to replace a toilet in about one to two hours.
Turn off the water at the valve behind the toilet. Incidentally, if you need to replace this valve for some reason, doing so when the toilet is out will make your work that much easier. It is also a great opportunity to repaint the wall behind the toilet. Next, flush the toilet. Hold down the handle to drain as much water out as possible. Now you can disconnect the hose from the tank. You may be able to do this by hand. Otherwise, use a pair of slip-joint pliers to loosen the nut. Have a small container ready to catch the water from the hose and fill valve. Because of the weight and size of two-piece toilets, it is advisable to remove the tank from the bowl.