Click into the field in browse mode, go to the menu Records -> Replace Field Contents... then look at 'Replace with Calculated Result'
Replace is a powerful tool and I use it a lot.
However, be careful using it on files currently being shared over the network. Just as in running SQL UPDATE in MS Access, if a user or users have locked some of the records, they won't get the update the other records did and you'll have a dickens of a time figuring out which records got skipped.
So use it, but with caution.