You've done all the work of animating the cube so it comes together. Now you'll reverse the animation so the cube explodes out of the comp at the end of the animation. Instead of repeating the steps you performed to create the first half of the video cube's animation, you'll duplicate the existing Position keyframes and reverse them in time. Here are the steps:
Select all the layers except Null 1 in the Main Timeline, and press P to display their Position properties.
Deselect the layers.
Click the first Position property's name to select all of that layer's Position keyframes, and then copy (Edit > Copy) the layer's keyframes.
Go to time 13;00, and paste (Edit > Paste) the keyframes onto the same layer.
Repeat steps 3 and 4 for the remaining layers.
Duplicating the first pair of keyframes on each layer
Use the Selection tool to drag a marquee around (appears as a dotted line as you drag) and select all the Position keyframes that appear starting at time 13;00 and later in the Timeline. To create a selection marquee, make sure you click somewhere other than on a keyframe or a duration bar.
Dragging a selection marquee around the layers' last pair of keyframes
With all the layers' last two Position keyframes selected, choose Animation > Keyframe Assistant > Time-Reverse Keyframes. As a result, the box appears intact at time 13;00, just as it does at time 2;00, and it appears exploded at time 15;00.
To increase the speed of the box's explosion, use the Selection tool to select all the Position keyframes at time 13;00.
Go to time 14;00, start dragging the selected keyframes, and then press Shift to snap the keyframes to the current time (14;00 seconds).
If you press Shift before you begin dragging a selected keyframe, you'll deselect whatever keyframe you clicked on instead of snapping all the selected keyframes to the current time.
If you expand the Null 1 layer's Transform group in the Timeline and change the values of its Orientation, you'll notice that the box sides remain in cube-formation whether the box is exploded or intact and that the box faces a different point in space according to the Orientation values you specified.
The box at time 11;03 with the Null Object layer's Orientation set to 92, 179, 183
Make these final adjustments:
Set the Spinning Cube Pre-comp layer's Scale to 75, 75% and Position to 360, 208.
Turn on the layer's Continuously Rasterize switch in the Timeline's Switches column.