You could try to soak the foot in warm water w/ Epsom salts, dry it thoroughly then apply Neosporin.
After that, wrap the foot with VetWrap (stretchy sticks to itself bandaging).
Do this daily-- you may have success, as many other people have.
If you don't see improvement in a week or so, you will need to remove the infection by surgery.
Here is good info for you:https://www.backyardchickens.com/t/236649/bumblefoot-surgery-with-pics-and-how-to
Credit to BYC member ruth.
We had a chicken with this exact problem!
1. We took out what was in her foot to start!(looked like a rock)
2. Then we washed it with just normal hand soup!
3. We then put Neosporin all over the infected area!
4. We wrapped her foot with gauze and secured it with medical tape!(not on her skin)
5. We isolated her with food and water!(so the other chickens would not pick on her)
6. We put B12 in her water for about a week I think!(look up dosage to be exact)
Everyday or every other day we took the bandage off and soaked her little foot in warm water with soap for about 5-10 minutes! And put more Neosporin and wrapped! Her foot is perfectly fine now!
I hope this helps!
Do your reasearch though because this method helped us but maybe not for others!