How large is the tank? If it is a cycled tank (5 gallons plus with a filter), partial water changes will suffice, but if it is uncycled, smaller than 5 gallons and unfiltered, you'll need t0 do 100% changes.
For a partial water change, you use the vacuum to siphon out the water as well as the dirt, then replace it with conditioned water of the same temperature.
For a 100% change, you remove the fish and put them in their bags or cups, then empty and rinse the whole tank. You then fill the tank up again, replace all the decorations, turn the heater back on and allow the water to get to the proper temperature. You then float the bettas to re-acclimatise them to the new water. :)