Hi Mari! My name is Maddy and I work for an outdoor gear repair shop in Denver, Colorado.[br]
I would sew a folded-over layer of fabric over the top portion of the mesh (on top of where it detached from the jacket), and then reattach the mesh to your shell with a line of stitching over your new fabric addition. Putting in the extra layer of fabric at the top can give the mesh more structure as you’re trying to reattach.[br]
If you’re worried about waterproofness in any way, I would recommend getting some seam tape to iron onto your seams.
I hope this can help, I’m interested in hearing how your repair turns out. Best of luck!