Get a siphon/or any plastic tubing. Stick one end in the water, other end over a bucket. Suck the bucket end quickly, water will start draining out into the bucket When you have taken as much water as you wanted to, lift the tube out of the water and it will stop. Then fill up the tank with water that has been dechlorinated Takes less than 5 mins for a small betta tank
For a 1 gallon I take the fish out and put him in a cup of water from the tank and then I rinse the gravel and the tank in the sink. Once it’s all clean I fill it back up and let it adjust to the temp then put the fish back in. For my larger tanks I use my gravel vacuum but I don’t bother with a 1 gallon tank
Here’s how I do it: I use a siphon that connects to the sink. Drain 25% of water, make sure to suck up any mess from the substrate, and wipe down glass if necessary. Then I add the recommended dose of Prime, set the water to semi-warm, and fill the tank back up. Easy aha.