Commercial (and intro) markers live in the media_parts table, in the extra_data column; the data in this column is URL-encoded. However, this table doesn’t hold the actual name of the media to which the data belongs; to get that, you’ll have to use the media_item_id to join on the media_items table. And from there, you’ll need to use that table’s metadata_item_id column to join on the metadata_items table.
Editing the data in media_parts to adjust the markers probably isn’t practical. I agree with @pl_5309, the best way to correct your commercial markers would be to configure the library for ad detection for all items and then perform the Plex Dance on the affected items. After doing so, forcing a media analysis should cause ad detection to run; I’d wait until intro detection for the items has completed, however.
