If you're comfortable working with spreadsheet programs, you can export your item file, then open the file in your favorite spreadsheet program, and filter the data so that all of the items of interest are grouped together (I'm assuming this can be done.)  Then you could easily change all of the fields that you want to, and then import the new file back into Square.  I would suggest keeping a backup file of the original exported file, though, because you can't be too careful.