There are at least three possible meanings for "reverse" as it applies to an image in Photoshop.
1)
Revert to a previous state. CMD-Z, Edit-> Step Backward, or clicking
one of the previous states in the History palette will allow you to
reverse an action you've just done. It also helps if you use layers to
do your work so you can easily return to your original.
2) Mirror
image. To flip everything on the file choose Image->Image
Rotation->Flip Canvas (Horizontal or Vertical). To flip a layer use
Edit->Transform->...
