You can drag clips in the Project browser to rearrange them however you want in your project.
To move a clip in your project:
In the Project browser, select one or more clips, and drag them to a new location in the project. A vertical green line appears in places where it’s OK to place the clip.
If you have a Mac with a trackpad that supports Multi-Touch technology, you can move a clip to the right or left of the clip next to it by selecting it and swiping three fingers right or left on the trackpad. You can’t swipe a clip past a transition.