Since your profile says that you are unaware of the nitrogen cycle, I'm going to assume this tank is not cycled. Does the tank have a filter?
Either way, I would recommend doing 50% water changes every other day using SeaChem Prime as your water conditioner. The reason I recommend Prime is because Prime will detox ammonia for 24-48 hours. Once you understand the nitrogen cycle, you will learn that fish constantly release highly toxic ammonia into the water. In a cycled tank, that ammonia gets turned into highly toxic nitrites, then the highly toxic nitrites get turned into far less toxic nitrates. Then in a cycled tank, you normally will do a weekly partial water change to keep the nitrate levels low enough to be non-toxic.
I highly recommend you read up on the nitrogen cycle, since it is the most important thing to understand when keeping fish. Betta's (like all tropical fish) require a heated, filtered, cycled tank. Sadly, most fish stores don't inform their customers of this.