Pedro, the idea they are trying to tell you is to build a timeline with a different layer for each photo.
6 photos = 6 layers. Your final timeline would start out looking like a staircase, where layer 1’s content ends, layer 2’s contents start, and so on. Then if you want to swap 2 and 6, you simply drag 2s content down its section to where 6 starts, and you drag 6’s content the other way to where 2 used to start.
I would also suggest playing with drop zones. Depending your your layout, a different drop zone for each photo can be easily changed from 1 photo to another.
You could also go into your media managment tab, and call up photo 2, and tell the program to use the file for photo 6 here instead.
Hope this helps.