Suppose battery of drone will discharge then drone will fall down. So,what is the solution for this problem. How we now the battery is low so land your drone

Advanced controllers like PX4 and APM have failsafe options where it returns to home on battery crossing a certain percentage such as 20% which is modifyable by the user.

Suppose we have KK 2.1.5 then what to do

This will help you,
https://robu.in/product/lipo-voltage-checker-28s/?gclid=Cj0KCQjwkoDmBRCcARIsAG3xzl_EsnIWGBlYzfIc-Qq1Vm2b_FjBzvvmoG7tvD98WI6ZBjm9ppkVdVgaAj1uEALw_wcB

I already had. But what to do when drone is flying and on that time the battery will discharge.

If you don't have advanced flight controller like Pixhawk etc, to find out flight time, make rough calculation with drone weight, motor current and battery capacity. Then fly your drone at 6 ft height to check the calculated flight time with LOW Battery indicator. Flight time will not be constant as the drone flies at high will experience wind load, frequent maneuvering will drain the power quickly. You have take these factor in to account while flying drone without low battery safety measure.

In addition to that, if you use same battery for more times, LiPo's will degrade with no. of cycles and time. You have to take this too into account for safe flying.

Ok thank you so much