In a three phase motor direction of rotation of the motor can be reversed by changing _ of the three phases?

The windings in a 3 phase motor, when activated by a 3 phase supply produce a rotating magnetic field in the rotor area of the motor. Swapping phase A with phase B re-orders the fluxes so that the flux rotates in the opposite direction. Swapping B with C does exactly the same thing as does swapping A with C. Think of it like a triangle with corners called A, B and C. If you swap any two corners and follow the points A, B and C you'll go in an opposite direction. Swap two more corners and you're back to the original rotation

Just interchange of any 2 Phase by manually or automatic control.

by changing any of two phases with each other

swap any two phase connections of motor.

yes just by change two phases 

by changing the phases the rotation of the motor can be reversed, since the rotating magnetic field is reversed

Just interchange the phases RYB to YBR

Note: Applicable for only Induction motor 

The three phases applied to the motor produces the rotating Magnetic field which tends to rotate the rotor of motor . if we swap only two phases the rotation of Magnetic field will be in reverse direction in result the rotor of the motor also change the rotation direction .

By changing any of two phases, so that it the rotating magnetic field is reversed to result in reverse of the rotation direction

the order of the three phases

by changing its phase sequence ( or reversing its phase sequence)