To turn it off, the most straight forward way is to go here.
Don't worry if you do not have the tab. If you don't, it means autoplay does not happen for you.
Remember, on your mobile phones, there is also an autoplay setting which you need to turn off.
For Apple products go to iOS settings, not Facebook. Scroll down to under app settings and then click Facebook. Click settings. 2nd section down there is choice for autoplay. Click it and change to off.
For Android, go to Facebook, click on settings. Scroll down to under app settings (Not Account settings)
Click on a section on Video-Autoplay and click it and change to off.