Fixing stairs to second floor

[Jbois03]

Hello! We recently bought a 110 year old house. The stairs going up to the second floor are cracked in multiple locations, along the threads and the walls and they sag a tiny bit when we step on them. We have access to underneath the stairs, but would have to tear through the ceiling to get there. Aside from doing that, is there something we can do to fix these without hiring a contractor?

thanks!