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You do not need to do a major cleaning unless the bowl has developed slime or grime you cannot scrub off, or if the water has become permanently cloudy. The more involved the cleaning is, and the more water you replace, the more stressed your fish will be. If the fish bowl is an adequate size for your betta fish, at least 3 gal. (6 L), you will only need to clean it this way once or twice a year. You'll be replacing about 50% of the water in the fish bowl. Use tap water treated with dechlorination tablets, freshly collected rainwater, or distilled water treated with aquarium salts. Check the water temperature to make sure it matches the water in the fish bowl. See the preparation section for more information on preparing aquarium water. Move roughly 50% of the water in the fish bowl to another container. Even if the water looks filthy, do not transfer your betta to a container of new water. The shock could kill it. Keeping the old water also insures that you will still have useful bacteria in your fish bowl, which will break down otherwise harmful chemicals produced by your fish's waste. Separate out any gravel, pebbles and decorations by pouring the water through a sieve standing on a bucket, or by scooping them out with a cup. Dump the remaining water into your garden or down a drain. If your bowl is too heavy to handle easily, use a siphon hose instead to transfer the water to a bucket in stages. Rinse all gravel, pebbles, and decorations once in a basin of hot water to remove as much grime as possible. Pour out the hot water and rinse a second time in cool or room temperature water. Rub them with your fingers or your dedicated aquarium sponge while rinsing the second time. If water and elbow grease alone won't remove the gunk on the fish bowl, wet a cloth with white vinegar and scrub the sides. Always thoroughly rinse the vinegar in room temperature water after the fish bowl is clean. Never use any soap, cleaning products, or any substance besides water or vinegar to clean your fish bowl. Return the gravel, pebbles, and/or decoration to the fish bowl. Pour in the newly prepared water. Pour in most of the old water, but leave enough in the temporary container for your betta to swim. Wait for the clouds and gravel to subside to the bottom if necessary. Because of the major changes to the bowl, introducing your fish slowly is recommended. Float the betta fish and the water it's been swimming in inside a small plastic bag, on top of the bowl water. After 15 minutes, pour a little of the bowl water into the bag. After 30 minutes, let the betta swim out of the bag into the rest of the bowl, and remove the bag.

Summary:
Use this method only when absolutely necessary. Prepare a new container of water. Transfer half the water and the betta to another container. Empty the fish bowl. Rinse the contents twice. Scrub the fish bowl with vinegar if necessary. Set up the fish bowl again. Return the betta fish to its bowl.