Volkswagen: Oxygen sensor light, maintenance reminder, oxygen sensor

Question
I have an 86 cabriolet with 30,000 miles. I bought it last year at 25,000 from a 91 year old lady and it is in show room condition. Last week the O2 sensor light came on so I went out and replaced it. The light is still on. The manual(which I should have read first) says its a maintenance reminder and to take the car in for service. I've been through the car from one end to the other and everything from filters to spark plugs is new. Last emission test showed almost no emissions. How can I rest the sensor to turn the light off and save an expensive trip to the local dealer?

Answer
it may be that the only way to reset the service indicator is to get it pluged into a computer. there should be other alternatives to main dealers, i will try and find out if it is possible to do it any other way and let you know. ps, sorry about delay, moved house and my computer blew up.