I am trying to speed up a small part of my magix video but I am not sure how to do this. Also how do I add music to a certain part of my video too please
Speed up & down can easily be applied using keyframes and setting the speed at each keyframe. You can watch this video about keyframe as it betters show how to use it
The youtube video is about brightness but you just substitute using speed in your case.
For music, best to go to timeline mode and drop a music mp3 into one empty timeline. You can grab the ends of the music object and shorten it to the part of music you want to listen to, and move the entire music object to the part of the video you want to playback with the music.