Do you plan to use what's in the tank now? If so, it's pretty simple. Pre-plan everything, and be sure you have everything you'll need to make the transition. Make sure you have enough fresh saltwater mixed ahead of time. If you have fish, put them in buckets with water from the tank. An air stone will be handy if they'll be in the bucket(s) for several hours. If there are live rocks and sand in the tank, drain enough tank water into a container large enough to hold the rocks so they are submerged. If you're re-using the same sand, it's a good idea to remove everything first, the give the sand a good swishing to get as much nasty stuff loosened up, while siphoning off the water. Once the water has been drained, scoop the sand into a separate bucket, or buckets. When the tank is empty, simply swap tanks. Put the sand and rocks in the new tank, lay a piece of plastic or a large plate over the sand (reduces sand storms), and fill the tank with what's left of the old water (not the dirty water from swishing sand!), and finish filling with the fresh saltwater. It's a good idea to acclimate the fish like they are new arrivals, as the water parameters will have changed a bit. If you pre-heat the fresh mixed water prior to starting the project, it will speed up the process by not taking as long to reach the desired temperature. The water in the buckets will cool without heaters, as will the water when added to the cool, empty tank. If it's possible, try to keep the water in the buckets at the same temp as the tank was when everything was before starting the project...