If your tank is cycled you need to do 50% weekly water changes with gravel vacuum, filter media rinsing in the old water WITH a water change, walls cleaning. If there is any uneaten food from him always remove it from the water never let it sink to the bottom.
I am going to give you a few links from Oldfishlady about how to take care of the cycled tank and water changes.
Or if you don't want to cycle you can do 1-50 and 1-100% water changes every about 8 days. But you have to acclimate your betta properly to the new water