The easiest way to make a transparent background is with the help of the Wand tool. Load the image, then press Wand and adjust the tolerance to 25 %. Click on an area of the background and it will select all the same color. Press Ctrl + X to Cut and repeat the step until you get only the object you want with the transparent background. Go to File > Save as and save the image as PNG. PNG format keeps the transparency. To change the background with anything you want, add another layer and import the picture, then drag the first layer with the object above the background and that's it.