With the sensor free, take your rag and wipe the sensor until its clean. I prefer not to use any chemicals on the sensor to avoid potential problems. If you need to, use mild soapy solution and rinse well. The ABS sensors are precision instruments in a crude environment. They're tough enough to hang off the brakes of a very fast moving vehicle, but one good knock and they could be damaged beyond repair. Keep this in mind when working with these sensors, They're tough, but a little car taken while you service the antilock braking system along the way can save you from another costly repair added to the brake service.
To finish up the job, reinstall the sensor in the same manner it was removed, taking care to install the sensors in exactly the same manner as they were removed. Don't skip the step of reattaching the line or wiring to those mounting points. They may seem like they don't matter, but it can get very expensive if you make a bad decision.
*Don't be discouraged if your ABS light doesn't turn off right away. It may take up to a few days for the system to reanalyze itself and reset fully.