When creating a Campaign Template, you have the option to include Dynamic Creative. This means that you can upload more than one video or photo, and more than one version of your text copy. Facebook will automatically pair one piece of media and one version of the text copy together, and optimize for the combination that performs the best.
You'll see options to add multiple text options and video or image options on the Creative screen of your Campaign template or Ad setup.
Add up to five text and/or five media options. Once you've added your options, click Save. You can always edit your options, which makes it easy to change copy and creative on the fly. Keep in mind that any of your media items could be paired with any of your text copy options.
To edit your creative after the ad has been published or scheduled, click the three-dot menu at the top right of your ad in the Campaign stream, then select "Dynamic Creative..."
You will be able to toggle between the text and media tabs to make adjustments to the creative.