Quad turns in reversed direction to yaw stick

Perhaps a strange question as I have been flying for a year now.
Right from the first flight Roll, Pitch and Throttle has worked as it should. Likewise arming with throttle stick down and to the right (mode 2, I think), but I have noticed that when I press the yaw stick to the right the Quad rotates to the left and vice versa. Have noticed that, but not bothered me so much until I started with FPV. Still lots to learn. Have read the information on Arducopter and that PWM should be low when it should turn to the left. PWM is low when I move the stick to the left. So, what is wrong?
To reverse channel 4 in the radio does not work, as when I do, I have to arm with the stick down and to the left.
There must be some other way, which I’ve failed to find out. Should appreciate some help.