Open your source video, then select Picture Settings. The following is a screenshot from OS X, but it's similar on Windows.
Your source is 1280px wide. You need 960px, thus 320px to get rid of. This means you want to crop 160px at each side of the video.
Choose your export options as you like, but you should know that by doing this, you will lose a fair amount of video quality, as the video will be re-encoded. This is why you should probably use the highest quality settings possible (i.e. the High Profile, with h.264 as codec.
Also note that cropping and destructively editing a video because it was poorly encoded (i.e. with black borders, "pillarboxed") is probably the wrong approach. Consider switching to a playing software that allows you to selectively crop the video, such as VLC.