To preview your 1SE video tap the big Mash button at the bottom of your project's screen. 

Android:

This won't save your video just yet. 

You will be presented with a preview of a video containing all the snippets you've added this year, but you will be able to customize it before saving (Android / iOS) or sharing (Android / iOS) it. 

To learn more about how you can customize your mash, take a look at this article.

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