+1 to Micho.
If your tank is unfiltered, you will probably have to do a 100% change at some point, in which case you will need to cup your betta. Try to bring the cup up behind and beneath him so he isn't stressed by it, and get a good amount of old tank water in the cup too. When he is safely cupped and set aside, you can set about emptying the tank and scrubbing everything out with hot dechlorinated water (no soap) giving your gravel a good rinse. When all that is finished, you fill the tank back up again and dechlorinate the water. Let the heater do it's job and get the water back up to between 78 and 82. Then float your betta in his cup in the water and let him acclimatise, adding a bit of tank water to his cup every five-ten minutes. Then you can release him. :)
You'll have to do one of these 100% changes per week, if your tank is under 5 gallons, and one 50% change.
For a 50% change, stir up the gravel to get the dirt floating. Use a jug to remove half of the water in the tank (a gravel siphon is much better, though, if you have one!) Then put your new water in a bucket, trying to match its temperature to the tank, and add dechlorinator. Then slowly pour it back into the tank. :)
So, to summarise:
- 1-5 gallon tank, unfiltered = 1 50% and 1 100% change per week
- 1-5 gallon tank, filtered = 2 50% changes per week
- 5+ gallon tank, filtered and cycled = 1 25-50% change per week