Unless I'm missing something, there is no function to update your mods on the Steam Workshop.
I had to delete an old version and republish a new version for an update.
I've got no idea if Larian will see this topic, but PLEASE add an update function AS SOON AS POSSIBLE!
While you're at it, please look into why mods you subscribe to aren't placed in the right folder or why the game doesn't see the mods in the folder Steam chooses.
Not to speak for other people, but there are quite a lot of modders with potential who aren't doing anything with the Steam Workshop because of these two flaws.