How do i change this overlay to the one i downloaded?

Are you talking about the Error overlay doesn’t exist? Try refreshing your portal page. If you have the portal open in one tab and the art catalog open in another, the portal page weirdly doesn’t recognize overlays uploaded after the portal was open. Refreshing fixes it.

Also, make sure you have it spelled as you do in the catalog.

You need to download the needed overlays for this template and upload them as your own overlays with the same exact names.