Facebook has finally roll out the feature that lets you edit your Look Back Video, I am not too sure whether the feature has been rolled out to all users. Here is the direct link: https://www.facebook.com/lookback/edit/.
You will be able to choose a fix number of photos from the four categories of the Look Back video, namely:
![Facebook A Look Back Video - Edit Step 1 Facebook A Look Back Video - Edit Step 1](https://farm3.staticflickr.com/2813/12354487134_ccd98a9deb_c.jpg)