When you are animating it can sometimes be helpful to copy and paste, and reverse, frames of your animation to create a loop of your footage.
To do this, you first need to select all the frames you want to copy In the project timeline.
When you have selected your frames, right click on any of the selected frames to open the editing box.
Select COPY from the menu options, right click where you want to paste your frames and select PASTE from the editing box.