You need to get hold of 2 large plastic containers. THey're normally easy to get hold of, most cheap houseware shops would stock them,. and you can get some pretty big sizes. Make sure it has a lid.
Place the container in a suitable condition. Syphon out some tank water into one container, then take the filter media out, place it in the container. Ensure there is enough tank water to keep all of the media wet.
Syphon out some more water, and fill the other container. If necessary, syphon out more tank water, until there is barely enough water for the fish to swim upright in. This will make it fairly easy to net the fish. Do so, and place the fish in the second container, and put the lid on.
Remove the rest of the tank water, along with substrate, etc., and swap the tanks over. Set up the new tank with substrate and whatever. Put the media back in the filter. Fill the tank with warm water, roughly the same as the fish is currently in, turn on the heater. Net the fish, and put him back in the new tank.
Job done.