If you mean her legs don't straighten when she tries to stand, then frequent physiotherapy can help to straighten them, but only gradually. By frequent I mean three times a day, gently bending and straightening the knees, and ankles, while supporting the knee joint with warm hands.
However, there are other problems that can affect the joints such as 'joint ill' which is caused I think by a bacterial infection, so will need vet treatment.
Another possibility is that the leg has been broken or damaged.
Excuse me for asking, but why did you take on this lamb with such a problem? To my mind it's always dodgy taking on orphans, especially if you get them from a mart, or you accept them as a 'gift', because you don't know what the first few days of their lives have entailed. Were they fed colostrum? Were they fed properly in those early days? Were they stressed or did they pick up an infection by going through the mart? What is the health status of the flock they come from?