Assuming you have more than one tank, if one of your tanks is the temp you want, it's easy.
Unplug the heaters and let them sit in the tank with the right temp for about 1/2 hour. Plug one of the heaters in and turn the knob until the "pilot light" comes on. Back it up until the light goes off. Now do the same thing with the other heater. Unplug the heaters and put them back in your preferred tank. Plug 'em up.