How can I strengthen a torn acl?

You can't. All you can do is strengthen the muscles supporting the knee but it will never replace the function of the ACL and your knee will suffer.
You can't directly. When an acl is completely torn, it doesn't have the ability to heal or "strengthen" itself. However, by strengthening the major muscle groups around the knee i.e. Quads and hamstrings, some patients are able to compensate for the instability caused by the torn acl.