Most modern water heaters have a fitting to drain water using a standard garden hose - if you don't have one on yours, wait for it, your water heater will tend to give out every 10-15 years and you'll need a new one. Draining the water from the water heater is one of those annual maintenance things I mostly do almost every year (not as important as batteries in smoke detectors, so really ever 2 years as long as I remember to do it). Its important that you turn off the burner coils, turn off the inflow of water, open the water line to allow the tank to drain, then drain the water, then reverse the whole process. I'm told draining the sediment can extend the life of the water heater, who knows. My owner's manual has the directions, and in the era of youtube I wager you could find a How To video there too.

Its all kind of academic if you don't have your water heater reliably strapped to prevent tipping over - with a gas water heater this can be problematic, and the heater beomes another source of gas leaks after an earthquake. I upgraded the strapping around my water heater last month while replacing the furnace next to it, which suddently gave me access to a couple wall studs I couldn't before.