1. In theory, to just change a radiator, you turn off both valves, drain and remove the old radiator, fit the new radiator, reconnect the valves and fill the new radiator.
2. In practice, unless the new radiator is identical with the old one in terms of height above floor, width, and distance of pipe centres from wall, you will have to alter the pipework.
3. To alter the pipework you need to stop the flow of water. Freeze, bung or drain.