Editing Published User Generated Content
From Two Point Wiki
Users can edit and update a custom item’s preview image, title and description on the steam workshop. To edit a published item select the button next to the item you wish to edit in the items list.
Select the “Publish” button to open the “Share on Steam Workshop” menu.
Once changes have been made select the “Update Workshop Item” button.
Editing and updating an item can also be achieved directly through the Steam Workshop.