You can tighten up the muscles that surround the knee, which will help with the stability some, but the only way to fix the ligament, is surgery...
Been there. I demolished my ACL in 03, and we tried to control the damage and stave off the surgery by doing lots of leg strenthing exercises and therapy. In the end, it didn't help a lot...too much damage. I had my ACL replaced 2 yrs ago, he did a meniscotomy, and took out MOST of the medial meniscus, so I'm almost bone-on-bone, and will probably need a full knee replacement within a couple of years. ...<be still my heart>
The knee has been more painful and more trouble since, than it was before it was "repaired"
But your experience may vary...