Look under the tank wich is under the seat and see if you can spot the drain hose for the tank which is usually mounted to a bung that is part of the frame. You remove the hose and clamp if there is one on the hose and drain it from ther. Do it nice and hot because those tanks hold some sludge in the corners.
Don't be concerned about the engine, there really isn't that much in the engine on that bike. If you really have to to it then drain the tank and add a quart to the tank and run the engine for a few seconds and drain it again, then replace the filter and complete the oil change.