Turn the amp off and unscrew the back panel using a Philips screwdriver. Let the amplifier cool for at least 15 minutes if you were just using it because the tubes get extremely hot and could burn you if not sufficiently cooled.
Locate a black rectangular plastic plug cover found in between the preamp and the power amp. Lift the plastic plug up to reveal the bias screw.
Equip a pair of latex gloves and pull out the tube from its socket in the preamp. Attach one of the three probes of the bias probe onto the tube socket in the preamp, another to the connecting pins on the tube and the third to a digital multimeter.
Set the multimeter to "Resistance" and turn the amp on. Examine the multimeter reading. Consult the amp's user's guide for the nominal and maximum bias voltage. Turn the bias screw clockwise if the multimeter reading is below the nominal bias voltage and turn it counterclockwise if the multimeter reading is higher than the maximum bias voltage.
Set the bias in between the nominal and maximum bias and turn off the amp. Remove the bias probes and replace the tube into the socket on the preamp.