What state is the Clip and the Project in? If the Audio & Video have been Exported to a delivery format, and that is all you have, there is not much you can do, except "cut" into each the Audio & Video and edit in a different manner. Do not forget that you will need Handles to effectively use most Transitions.
Now, if you're still in the Project, you can undo either. The exact method might differ slightly depending on exactly how these Fades were applied. If they are Dissolve Transitions, just click on the Transition and hit Delete. Make sure you are ON the Transition and not the Clip.
If these were done via Keyframing, you would Clear the necessary Keyframes.
I hope that your instance is the latter, and not the former.