Update website with 0 Downtime

Hi there,

I’d like to know if there’s a tool i can use to help in the deployment of my websites. i know about bitbucket and so forth, but am looking for a way that when i need to update custom themes/plugins, i can do that without any downtime. eg. sometime am on facebook, or even wpmudev website and design structure would change without any form of downtime. what’s the secrete lol.