You can use any one of the fill methods to change the background color. (shift-F5 is the fill command) The most common method is to use the two swatches just below the toolbar on left side of screen. Take a look at your keyboard at the right side of the space bar are two modifier keys alt or option and ctrl or command picture the left key at the left swatch and the right key as the right swatch. Hold down that key and at the same time press the backspace key. Your document or selection will fill with that swatch color. Double clicking on either swatch will bring up the color picker to choose a different color.
If there are other objects on top of the background and they are merged with the background, it will be necessary to select the background using the selection tool in the tool bar before filling the background with a color.
Hopefully this helps, if not post a picture of your screen so we can see what your trying to do and we can then give more advice.