step one get a filter if you don't have it.
step two place tank on kitchen table
step three place bucket on floor.
step four get gravel vac or hose 4 - 6 foot long
step five take betta out place in old tank water in a cup
step six place vac end or one end of hose in tank
step seven crouch down and suck opposite side of hose/ vaccume until water is about half way down to you. quickly complete next step
step eight either place or point your end *the end not in the tank* at or in the bucket.
step nine keep siphoning end of tube as close to substrate as possible untill all water is sucked out.
step 10 after water is removed dump old water put up all gear scrub tank with non clorinated water.
step 11 refil tank acclimate betta to new water and add him to tank.
if i may ask do you have a filter and heater for this tank?
if not i suggest getting a little air powered filter from tetra it is 12 dollars rated for 3 gallons or one betta. the replacement pads are about 3 or 4 dollars for 2 in a packette. so the total investment isn't alot of money. it is better for your fish and less time consuming than 100 percent water changes.
if you have a heater and a filter feel free to disregard. also if you don't have a heater a desktop lamp with a incandesant bulb will keep your tank warm enough.