That’s not really what Picard is optimized for, but you might be able to (mis)use it for that. Given you mentioned iTunes, I assume you’re looking at the song title rather than the file name? Otherwise, I’d recommend searching for “Mac batch rename” – you could technically do that with Picard as well, but it’s even more of a hack, especially when there’s almost certainly plenty of programs designed for that (at a brief look, this seems to do it well enough).
The simplest form of changing the field would be:
That will change every instance, though, so if for whatever reason you have that text later in the title, it would be better to explicitly specify the start of the string (note the second
r in the function name, and the
^ in the search string):
Whichever one you want, add the line to a new script in the proper page of the options, import the songs you want to change (100 should be all right, but if it goes slowly, do it in smaller chunks), and then save them. If the files are already matched to releases in the database, then you may want to drag them back to the left pane to avoid unexpected edits of the rest of the data. If you haven’t yet matched them, avoid clicking “Scan” and “Lookup” for the same reason.
Really, though, Picard (and, to a lesser extent, MusicBrainz as a whole) is about adding a broad variety of high-quality data about a particular track, and using it to simply change a single field is actually harder than it would be in most other programs. You might want to look into Mp3Tag on Wine, Puddletag if you can get it installed (for anyone using Linux, it’s an amazing re-implementation), or something else in that vein.