The main purpose of this brief blog post is to emphasize the importance of doing a complete backup, or two, prior to actually doing the upgrade.
Over 99% of the time the upgrade will go absolutely fine. But once in a while that's not the case and things will go horribly wrong. That is when you will be glad that you took the time to make those backups. There is nothing worse than being called in after something like that happens and explaining that someones pictures of their kids really are gone forever.
Even if your backup goes just fine, having a current backup can save you from other incidents like a lost or stolen computer.
So in the article below when Apple calmly says, "Before you upgrade, we recommend that you back up your Mac." Take it to heart and make it happen. Using an external drive and time machine is probably the easiest but there are cloud solutions like Crashplan as well. Personally, I do both.
Upgrade to macOS Sierra.
Here is Apple's article on how to upgrade macOS with a lot of good information like which models of Mac are supported and which features require newer hardware.
Update iOS software on your iPhone, iPad or iPod touch
The same thing about backups applies to upgrading iOS. Make sure you are either backing up to the cloud or to iTunes on your computer.
Update the software on your Apple Watch
Backing up your watch isn't really something that you need to do if the other parts of your system, the iPhone and your computer are backed up. But here is the Apple article