If a lens has a fixed aperture you cannot change it. But such lenses aren't very common for interchangeable lens cameras. The most common lens of this type would be a "mirror lens", and these are usually fixed at f/5.6 or f/8 for focal lengths around 500-800mm.
If your lens has a maximum aperture of f/2.8, you almost certainly have a constant aperture zoom lens that is capable of maintaining the same maximum aperture (minimum f-number) throughout its focal length range. Constant aperture lenses can be stopped down. The numbers in the model names indicate the maximum size of the opening, not the only size of the opening.
If there is no aperture ring on the lens that just means you must use the camera's controls to change the size of the aperture in the lens at the instant of exposure. No matter what aperture you or the camera select for exposure, the lens normally stays wide open until the instant before the picture is taken. This allows for a brighter viewfinder and more sensitive and accurate autofocusing.