A poorly positioned subject can ruin a great photo. MacLife walks us through a few tools that can change the position of a subject.
You have two options: Use Photoshop's Clone Stamp tool, which allows you to "re-paint" the subject into the new position, and then select the background to paint back over the old subject. With the Content-Aware Move tool, you select the entire subject and move it into its new position. Then you use the eraser and spot healing brush to tidy up once you're finished.
Neither tool is that much more difficult to use than the other. The Content-Aware tool can require some erasing and spot healing at the end, but the Clone Stamp tool might affect your pixels.
If you want to see the steps in action, Mac|Life moves a cow using both methods described above. Hit the link below to check it out.
How to Reposition a Photo's Subject [MacLife]