A water change is basically removing water from the tank, then adding back the water you removed.
The simplest method is using the Python No-spill. It attaches to your faucet and allows you to drain your tank and then fill your tank up within a few minutes. Otherwise, you can do the old fashioned method of just getting a syphon and bucket and drain some water from the tank into the bucket. Then dump the water into the sink. When you removed enough of the water, you fill the tank back up again, using the bucket again.