I don't have any info about editing subtitles but downloading subtitles for an entire series is pretty easy.
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There are multiple tools to edit subtitles. From the web based https://subtitle-editor.org/ to really powerful software like SubtitleEdit
Depending the format... .ass
files can be edited by aegisub mostly used by fansubbers in the anime world.
If you have the opensubtitles plugin set up with an account, you should be able to manually run the "download subtitles" task in the scheduled tasks, and this will search for and auto-download subtitles for your entire library. That's the best way I've found, can't speak to editing subtitles that are downloaded. What kind of editing are you trying to do?