I actually use my mini siphon for the same tank you have and it works fine even with my betta in it, as well as a good amount of gravel, a cave, a plastic plant, a mini heater and the mini filter it came with. It does suck the water fast but not so fast that you can't control it. If you've never used a siphon before, just practice first in a bucket. I got mine from Petsmart.
Of course the other methods that OFL mentioned work well too.
(Can't wait to upgrade soon! Less water changes and cleanings!)