In this scenario, where you have accidentally deleted an update, you can reimport the update from the Microsoft Update catalog.
You'll need to use the WSUS native console, and you'll need to know some identifying characteristic of the update (UpdateID, KB Article Number, Title, Targeted platform). From the Updates node, right click and select Import Updates. Use the search feature on the Catalog website to find the missing update, load it to the 'cart' and checkout. The update will be downloaded and imported back into the WSUS server. For best results, you should perform this task from the local WSUS console on the WSUS server using the Administrator account.