2004 Vauxhall Astra SERVICE ALERT LIGHT

I have had my car serviced recently and the mechanic could not remember how to have the service alert light turned off. I was to call back at the garage but it is very awkward as it means taking a half day off work (since I commute from Omagh to work in Belfast and the mechanic is in Omagh).

Could you tell me how to swith the service alert light off so that I can do it myself and not have to take a day off work
Thanks