You have several options...
Here is a web site that will do it for you: http://jeeep.com/details/coord/
You can find software to do it using your computer. The best solution if you have lots of points to convert.
You can use you GPS unit:
Store the coordinates as a waypoint or landmark, switch the GPS to the desired coordinate system/datum, and recall the stored position.
This site converts between UTM and 3 lat long formats.