The simplest way to update OS X to 10.11.5 is through the Mac App Store: The Mac must reboot to complete installation of OS X 10.11.5.
Mac users can also choose to download and install OS X 10.11.5 by using a Combo Update, or the standard update.
Separately, Apple has released i OS 9.3.2 update, watch OS 2.2.1, tv OS 9.2.1, and i Tunes 12.4.
If you have a Time Machine backup, you may believe you’re safe but I would argue that it is not enough to EASILY recover if something goes wrong.
With a full system sofware change, it is quite possible that something may go wrong. Then upgrade to the latest versions of all your applications on your current system.
I believe it’s a good idea to stay up-to-date on Mac OS upgrades…but only cautiously with excellent backups in place BEFORE upgrading.
Yosemite is FREE and it’s been out for a while…so it’s somewhat of a no-brainer to move forward. If you use an i Pad with sync to your Mac or anything online you want to be on Yosemite.