Suppose there's a thirty second audio clip ... and a completed slide show track that's ten seconds longer.
First, you might lock the audio track (lock symbol at the left), to make certain that it's not affected. Then select all the video tracks, right-click and GROUP SELECTED CLIPS.
Now place the mouse pointer at the end of the video track until it becomes a ] bracket ... and drag left until the end of video shrinks to match the end of audio, shortening the length of each slide proportionally. Video can be lengthened (stretched) as well, using the same method.