Changing multiple fields at once

[ohrenkino]

see e.g this thread:

Copying Artist from line 1 to all other lines

I would use the tag panel. Select all files in the file list that should get the same data, turn to the tag panel, and the field that is to be updated, select the already existing data from the dropdown list press Ctrl-S to save the modification. For fields that you do not see in the tag panel, use the extended tags dialogue (Alt-T). There is no way to do it with an action.