If the legs have already died like in the pics I sent, there isn't anything you can really do unfortunately, especially with large birds. They'll eventually self amputate at the point between healthy and dead tissue, leaving just the stumps. Some people try to get or make prosthetic feet for their birds.
I have a hen who only lost one leg last year and she does okay, but she's a light, maybe 3 pound bird, so there isn't a ton of weight on her remaining foot all the time, and she has found ways to rest and take some weight off that leg (like leaning against a beam enough to have her keel support some weight).
When it gets cold and snowy, I do foot checks with anyone I see acting even the tiniest bit off. If the foot is stiff or frozen solid, I'll take them inside and slowly warm the foot up (too fast and it damages the tissue more). Then you have to keep it clean and so far just kind of hope you caught it in time. I have a few birds that I was able to thaw their feet enough to save them, most ended up losing a majority of their toes though.
I haven't yet found out what is causing it other than them being able to walk in snow and it never happened before a few years ago. I know my birds aren't standing in water, they are covering their feet at night, they have access to dry places and aren't stuck somewhere with drafts.