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You can simply use the “Replace Field Contents” command with a calculated result; or fill in the value in one record, select the command while you're still in the field, then accept the suggestion of that value.
Make sure to Show All Records if you want to apply the changes to all records.
(This functionality is also available as a script step of the same name, but writing a script shouldn't be necessary here.)
The usual caveat: there is no Undo, so be sure to have a recent backup handy in case something goes pear-shaped.