just use a bowl (not too shallow) from the kitchen, make sure you have a book to cover it when he's in there.
fill it with water from his tank.
catch him with a net and gently drop him in there and cover the bowl. put him aside to a quite place. sunlight is handy to keep him warm but dont
use a transperent bowl then caus it will stress him.
clean his tank
fill it with fresh water and LET IT HEAT back to the temperature it had when you removed the fish (i use a kettle for heating up a lil water to accelerate that)
when the tank is settled you can GENTLY let the bowl with the fish into the water and let it flow into the tank. VERY slowly.