Coolant should be changed cold because when it's hot there's pressure.
If you pull the coolant pipe off when it's hot you'll end up with hot pressurised coolant spat everywhere including you.
Change the coolant by removing the drain plug on the water pump then loosing the radiator cap will enable the coolant to drain.
Drain the reservoir then refill at the rad cap , when it's full, run it up with cap off for a few minutes, let it settle and top up accordingly.
Once that's done top up your reservoir to the minimum level .