To change the background color of the picture you need to select the background pixels by using the Adobe Photoshop's Polygon Lasso Tool. The following steps will clear the process.

1. Start Adobe Photoshop and open your picture, which you want to edit. Please take the backup of original picture. Incase you don't like the edited picture; you can have the original version.
2. Now select the polygon lasso tool from the tool bar. If toolbar is not visible you can show it by using the menu bar. Window -> Show Tools.
3. There are three types of lasso tools available in the toolbar. Click and hold on any lasso tool icon. It will display all three, select Polygon Lasso tool.
4. Now you have to select the background. Click the edge between the object and background and keep clicking on all the edges. The click distance should be short. For good result you can zoom the image by pressing Ctrl and +.

5. Now select all the background. When the background is selected a marquee will be display the selected area.
6. Press Del to delete the selected pixels.
7. Select paint bucket tool.
8. Select the color from tool bar and click in the selection.
9. The background color will be changed.