Try Garmin's WebUpdater first. If that doesn't work splashy's source should but it's a little more complicated. There are instuctions in this forum somewhere but I don't have them handy...
Try this link for your unit.
Spoiler: Index of /perry/gps60
You might try here:
Spoiler: garmin 60csx broken? How to fix
or here:
Spoiler: GPSMAP 60CSx software version 4.00
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