My cycle is 26 days. Monthly I have brown discharge from the day 9 to 15 of my cycle. I need to know which day I ovulate. What is the cause of the blood.

Brown. Brown discharge is old blood, and it's possible that it's just blood left over from your period that's making its way out of your vagina at that time. Bright red blood, of course, is active bleed. Given that your cycle is regular, you could try ovulation predictor kit starting day 9-14. It's possible the brown spotting isn't interfering with your cycle at all.
If u ovulate. You would ovulate on day 12 which is 14 days before the end of the cycle, sometimes u can spot for a few days around the time of ovulation or could be from other causes. Please see your gyn.