Hello, after keeping my Betta in a 1 gallon bowl for about a year I have noticed that my fish is not very healthy and it even seems to have developed a mild case of fin rot. So I decided to go get an aquarium for my fish in order to keep it healthy. I bought a two gallon hexagonal aquarium that is made by petco and after reading up about aquariums on the internet I've realized I have no idea how to maintain it

So the questions I have are these:
1. The aquarium came with an undergravel filter and I don't know how to change the water or clean the gravel. With a 2 gallon aquarium do I change all of the water or just some of it?
2. Is 2 gallons too small of an aquarium?
3. Bettatalk.com says that undergravel filters should not be used for betta fish in small aquariums. Is this true?
4. Should I get a heater for my fish? Some websites say to keep the water at 78 degrees, bettatalk.com says room temperature is fine and a heater will kill my fish...
Any other tips or advice is greatly appreciated. Thanks for being patient with me