How to replace a igniter in furnace unit

First find the ignitor. Should be a black and white wire going to it. It will be located to one side of the gas manifold, in front of the venturi tubes. Get a replacement. DO NOT TOUCH THE SURFACE OF THE IGNITOR. Typically they are fitted in a harness and are held in place with a screw through the ceramic or metal .base. Some replacement units are wired to the old wiring with a wire nut after stripping the old wires. Run about $40 retail.