Batch apply Metadata to specific fields at the directory/Folder Level

Very often I need to apply the same metadata to a limited number of fields, to a large number of files across multiple subdirectories. Right now, I have to open each subdirectory and select the files then re-type the meta data. It would be much more productive if I could select the folders I want to edit and then edit a batch metadata window where blank fields would retin existing file metadata, and where I enter metadata, it would replace whatever data is in individual metadata fields in the individual files, across all the subdirectories.