Export
Under Export/Import, download all image metadata as CSV — or select images in the list and choose Export selected. The file includes an empty new_alt column intended for new alt texts.
Edit externally
Open in Excel/Numbers, fill in new_alt or change title, caption, description and status. Leave cells empty for fields that should not be touched — empty cells never overwrite anything.
Import with preview
Upload the file. You see exactly which images will change and how many rows are skipped before you confirm. Changes are then applied in the background.