how do I do an overlay to a video in the sandbox then export it so I can save it to a computer?

I have an iphone 5

im a rookie... lol I did 3 laps at Laguna Seca two days ago and used the app on my iphone 5 for the first time the videos started form the start/finish line after my 3rd lap we pulled into pit row and I hit something on my phone as I removed it from the car I only have 2 video laps that i was able t...