This is sometimes difficult when the clip is embedded as part of a sequence because it will change the position of other clips. It is better to isolate the clip later in the Timeline so that you can alter it and then put it back, repositioning the other clips around it. Unfortunately Windows Movie Maker only gives you the option to double the speed of the clip, so you must be sure what you are doing with it ahead of time. Double speed tends to make a clip far outside the normal rhythm of life so this must be your purpose ahead of time. It is also important to understand that this will significantly alter the audio track, speeding it up as well and raising the pitch. You will probably want to separate the audio track from the video clip and remove it otherwise it will take the full attention of the audience. This also works on photos, which are reduced from their original five seconds to two and a half. It is not advisable to use this on photos because you can drag or shorten the clips manually in the Timeline and that process ends up with a higher level of accuracy.