Apple's latest major desktop operating system, macOS Sierra bearing version 10.12, is now available for supported Mac computers as a free software upgrade. Updating your Mac to the new OS, you should be able to take advantage of the features Apple introduced such as Universal clipboard, iCloud desktop, Documents and Auto unlock that lets users automatically log into their Mac when they are wearing an Apple watch.
One of the most significant changes is Siri integration that brings all the features users enjoyed on the iPhone and iPad, along with a few more functions specially designed for use on desktop.
But if you like the older OS X version over macOS Sierra, you can always opt to downgrade using Time machine which is a built-in Apple tool on OS X that lets you back up all files to an external hard driver. You can easily roll back to a previous OS X version in case you come across issues after installing macOS Sierra, if you are using Time machine to back up your Mac.
It is important to note that reverting to a previous version erases all the changes that you have made to the files on your Mac after you installed the new macOS. To save the new files, move all of them onto a different disk or create a backup before downgrading. Check out the instructions to downgrade from macOS Sierra to previous OS version.
If you created a back up for your files, you can restore them. If you have used Time machine backup, then click the Time machine icon in the menu bar, select Enter Time machine and now you can retrieve your files.