Updating Workshop Items

First go to the Main Menu > Extras > Prison Sharing (Steam), click on your Published Prison Item and Play.
Then you go to the Main Menu > Extras > Prison Sharing (Steam) and you will be able to Update it. That's it.

If you are playing through your regular save file, you won't be able to update your Steam Workshop item.

Found many posts on the steam community, apparently the way to update your item is very different (depending on which workshop you are using).

For Skyrim, for example, the update is done using the Creation Kit. For Cities : Skylines you must use the Content Manager.

So in your case (prison-architect) it appears there aren't such managers, so no possible way to update the file, but you can still rename your old version to title[OUTDATED] and add a link in the description to your newly posted mod.

If anyone has another solution (than using managers) , feel free to correct me :)

Sources :

• Guide on how to update a mod for Cities : Skylines
• How to update a mod for Skyrim