Ohhh gotcha, sorry! lol
The reasons Bettas bite their own fins are unknown. Some say it’s stress, others say boredom and still others think it could be hunger, pent up aggression or even hereditary. The truth is, we just don’t know. There are a few things you can do to help him along. First, continue to keep your water very clean to avoid infection. Some Betta keepers have successfully broken the habit by changing the aquarium decor around or by adding more silk or live plants. Moving the tank to a new location or the fish to a new tank may help. Adjusting the light levels may be useful too reduce reflections which may fuel your Bettas aggression. If your aquarium is large enough, you may find adding a few peaceful community fish will keep your Betta occupied.
That is a bad habit and sometimes hard to break. I would try redecorating, moving around his plants and such every water change. As well as daily exercises such as a few mins in front of a mirror a cpl times a day. Poke a blood worm with a tooth pick and have him chase it around, etc.