Note which rotor is going backwards. Look at the bottom of the helicopter and determine which motor is hooked to that rotor. Follow your motor leads back to the control box ( 3 in 1 / 4 in 1 ) and turn the connector over. Note that both connectors are pointed the same way when they are both plugged in correctly.
That ought to fix your problem.

If you get both rotors going the correct direction but the tail (rudder) works backwards, pull both motor plugs and switch them.

Soloboss