Hi Yasir, Thanks for contacting us!
In order to change the background of an image you will need to use the Eraser tool. Start by opening up the background image, and then drag on your foreground image from the Photo Basket at the bottom of the editor. You can then use the eraser to delete areas from the foreground photo. We have a brief demo video which demonstrates how this is done.
Let us know if you have any further questions!