@sylviafors12137 there are a lot of different versions for the Series 158. It will help if you can provide us with the complete number for your sewing machine.
Check [https://www.youtube.com/watch?v=QPbEO2AseCI|this video] to see if it explains enough about the upper thread assembly. This [document|32755] may shed further light at the task at hand. Let us know if the model is appropriate for your machine. You can always post pictures of what you are up against and I am certain we can get this sewing machine back in working order. [guide|21499]
Amazingly enough, iFixit does not yet have a guide for this machine at all. While you work on your Kenmore, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.
Repair is War on Entropy.