A quick way to update a lot of rows

I've got a list of files in a mysql table.  One of the fields is the full path to the file (minus the drive letter). I would like to change the data in that field to just the top level folder rather than full path. Basically I want to get rid of everything after (and including) the first \.

The trouble is I have nearly 18000 records.

What's the quickest & easiest way to go about this?