Sounds like your doing too much like trying to change both the width and the height or not saving image.
Under menu item IMAGE find Resize/Resample. The second box down on the left has the current dimensions and pixels checked. High light the Width property and change it to what you want. The height will change automatically to keep aspect ratio. Don't mess with it.
Click OK at the bottom, if what you want save image under a new name to keep both the old and the new.