I've sharepoint 2013 farm last updated June 2014

First, verify you've installed SharePoint Server 2013 Service Pack 1. If not, do that first.

Then deploy the latest Cumulative Update -- do not just deploy security updates due to the gap. You also need a CU applied to be in a supported state by Microsoft. You must apply at least the April 2018 CU to be in a supported state, but I'd recommend applying the July 2020 CU which will cover all of the necessary security patches.