Why is your fish swimming with its back out of water?

The fish is DYING. The usual reason for this is bad husbandry. My advice is change its water ASAP then follow the rules. The rules of fish keeping are as follows. :- 1 inch of fish needs at least 1 gallon of water. :- Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% pf its water changed every week. If you keep to the above rules, and in the case of a Betta, keep its water at around 78F., and feed it good food it will stand a chance of living a normal lifespan. If you fail in any of them I can guarantee that your fish will be constantly getting sick and will have a very short lifespan.