Flip position values

The X-axis is left and right, and the Y-axis is up and down. I'm not exactly sure what you are trying to accomplish.

That looks like a screenshot of the graph editor with separated dimensions. The movement of the layer is originally right, then left, then right again. I don't know what the vertical movement looks like.

You can flip the graph upside down and change the X movement so that the movement starts where it now ends and ends where it originally started by selecting all keyframes and then choosing Animation/Keyframe Assistant/Time Reverse Keyframes. The problem with separated dimensions is that you will completely change the actual path of the layer by only reversing the X position keyframes. You may want to turn off separate dimensions. Changing the motion path in a predictable way gets a lot more complicated when you separate dimensions. You lose Bezier handles in the Comp panel, and modifying a curved path becomes almost impossible.