after changing a glowplug how do u reset the check engine light

The conventional way is an OBD ... The down and dirty way is to disconnected then reconnect the battery after about 10 seconds

Sometimes disconnecting the battery(s) will do it. You could drive it by your local Autozone or similar parts store and ask them to reset the code. They usually do this for free.

Some AutoZones won't reset alarms ... I asked ours why they will read them but not reset them and they said "Liability"

I agree with Js08016 about ignition cycle reset. I had the similar situation only I didnt replace the glowplug since I believe it was a faulty code do to block heater electrical interference. Checked the code with my scanner but didnt clear it just to see if it was a legite issue. Light stayed on for two days then just went away. Hasnt come back even at air temps of -10°F!