Hi everyone!
I have hiller 525 kit with sunny sky 980kv motors and kk2.1 kit.I am experiencing a typical problem. When ever I apply yaw, my quadcopter gains height. Else it flies very stable. Can anyone please guide me. Thanks.

This is very normal for a quad. (At least on the KK2) Go into the mixer editor settings and FIRST NOTE DOWN ALL VALUES.
Changing mixer editor values can cause your quad to go from very stable to very unstable.
So take a picture or note down on paper all the values before changing any value.

The mixer editor controls exactly how much each motor is sped up/slowed down when a particular control is applied.
Go to channel 4 (Rudder) and reduce all 4 values by a little.
Keep decreasing little by little  and test flying until the quad stops gaining height on applying Yaw control.

If something goes wrong, revert all values back to what they were before.

Thanks will try it right away.