Hover over it, and click on 'Unsubscribe from mod' (the red X button).
If you subscribed to a mod from the Steam Workshop, you can uninstall it from within the game: Instructions for uninstalling mods from the Steam Workshop: You might see a download progress bar in the list item if it takes a while to download. Return to the game, and you should see the mod listed in the Workshop mods list. Browse items in the workshop, and when you see a mod that you like, click on the 'Subscribe' button.
Instructions for installing mods through the Steam Workshop:
