Even if your server did not synchronize for six months (Mar 2013 - Oct 2013), any updates not acquired since March 10, 2013, would have been acquired on October 18, 2013.
You need to look at the Release Date column in the All Updates view, rather than the Arrival Date column to get an accurate perspective on what might be missing. Quite likely nothing is missing at all, because the synchronization architecture for WSUS is explicitly designed to prevent that from happening. By definition a WSUS synchronization retrieves ALL updates selected for synchronization that have been released since the last synchronization date, for each selected product category and update classification. (i.e. Adding a new Product Category will get you ALL of the updates for that product category regardless of any previous synchronization event.)
But to answer your question directly, if you needed to replace one, or a few, updates, they can be imported from the Microsoft Update Catalog using the "Import Updates" feature available in the WSUS native console. Open a WSUS console from the WSUS server, right click on the Updates node, select "Import Updates". This will open a browser session to the Microsoft Update Catalog. Select the update(s) you need to (re)import, and then click on the Import button from the "Shopping Cart". They will be downloaded and imported into the WSUS database. This process is documented in the WSUS Operations Guide in the article WSUS 3.0 SP2 and the Catalog Site.