You can take a shot from the Library (the source shot) and swap it with another shot in the Storyboard (the destination shot). You can replace the destination shot with a source shot starting at the first frame of the source, or you can replace the destination shot with a source for which you have offset the starting frame.
When replacing shots, you do not modify the overall duration of the timeline.
To replace a shot in the Storyboard without offsetting the source shot:
The destination shot is highlighted with an orange box.
The destination shot is replaced by the source shot.
To replace a shot in the Storyboard with an offset source shot:
The destination shot is highlighted with an orange box.
A dual view appears. In the left viewer, you see the original shot and in the right viewer you see the new shot.
The start frame of the first shot begins to change. You can see by how much you are offsetting the start frame by referring to the numbers in the upper-left corner of the screen.
You can also use the numeric keypad to change the offset. Specify a negative number and then press Enter to decrease the offset or specify a positive number and then press Enter to increase the offset.