How many gallons is this tank that the betta is in?? If it is too small, this prob. won't work too well.
When I had my Lionhead in a 2 gallon (Former inhabitants were picking on him), I knew I HAD to do water changes at least every other day. I couldn't reach in with a cup or anything to get water out cuz the fish would be in the way. So..I just got some air tubing (tubing for the air pump), cut to desired length and siphoned with that into a gallon jug. Depending on on the size of your tank, you can repeat as desired unless you got another/bigger container to collect the water. With the water being syphoned through such tiny air tubing, the water will come out really slow and it should not stress the fish out. You may even wish to use more than one of these tubes at a time to make it a bit faster.
Good luck.