To update an existing workshop item, click the My Uploads tab.
From there, you will see a list of your uploaded workshop addons. If you do not see any uploaded Workshop addons, please see the Publishing guide.
Find the item you want to update, and click on its name. An update dialog will open up, and will automatically populate with existing information for the addon for you.
From here, you can update the title, the image, tags, and the file. You may also provide a change log for your users to see what changed.
Once you are done with your changes, click update, and the editor will send the changes to Steam Workshop.
You may have to restart your Tower Unite game to see the update.