By selecting the background color you can change it using such tools as Hue/Saturation, Color Balance, Curves, Levels, in any of the color spaces that you might work in. The bigger questions is... How do I select the background? Again, there are five or six methods for doing this.
Visit Russell Brown's Tips and Tricks
and click through to more Tips. At the top will be a Quick Time tutorial titled Advanced Background Extract. Use can also build a mask to extract the background, but with newer options in Photoshop 7 they pulled that tutorial. A mask would work better, especially in the area under her left arm. If you are interested in the tutorial for masking email me directly.