First you need to make your canvas larger, do this by going to Image>Canvas Size. Enter your new width. You'll see a set of 9 squares with arrows extending from the centre: click on the middle row left square to add all the extra space to the right hand side of your document, otherwise it will put equal amounts left and right.You will probably want to use the Cloning tool to extend your image and you may have to resample a few times to get the best result. Select the tool, Option click (mac) or alt click (pc) in a section of the image you want to repeat in the extended area, then paint in the new parts of your image (you can enlarge the size as you can for any other brush).