Turn off the breaker supplying power to the attic fan. Disconnect the wires from the fan. If the wires are attached to the exhaust fan, simply loosen the clamp that connects them. Remove the screws that mount the fan motor to the housing. Pull the existing fan out.
Insert the new fan, then remove the regulator cover. Fasten and secure the regulator to a stud or rafter. Install a cable clamp into the hole, and work the cable through the clamp before tightening. Use a knife or wire cutter to strip the sheathing from the wires.
Follow the instructions for attaching the wires. In most cases, the color-coded wires will match exactly, such as white on white and black on black. The black from the fan may go to a red wire on the regulator, however. The green and copper wires typically will be connected to a ground screw, or else they will attach to each other. Always follow the exact wiring instructions that come with the fan, however.
Reinstall the regulator's cover after connecting all the wires. You should then be able to set the temperature control of the fan. While you can set this to whatever you want, most manufacturers recommend setting it to 95 degrees F. This provides a balance between cooling power and energy efficiency.
Flip the breaker back on, and wait for the fan to begin working. It may take a while if the attic is not hot enough, but you can still test to see if the connections work by turning down the regulator or waiting for the attic to heat up. If the fan does not come on, the most likely culprit is incorrect wiring. Check your instructions against the actual wiring connections you made.