How many bettas do you have?
A cycled 5 gallon is already pushing one-week water changes. A cycled 10 gallon would much better suit your needs.
Here's a quick link on cycling. If you stay with us and you actually begin cycling a tank (either fish/in or fishless, but that will be explained), then just ask ANY questions you have and someone will help you. It's hard at first, but it's easy to catch on.
For quicker water changes, you will want to (google this:) buy yourself a siphon
. Also runs under the names of gravel cleaner, or gravel vacuum. They're all basically the same thing though. If you youtube "siphon aquarium" you can see some demonstrations, although not all of them are correct/good methods.