Bumblefoot is a bacterial infection spawning from an injury to the foot. Do not attempt to invasively cut into the infected area. This can result in a more severe infection and the procedure will be very painful without the use of a local anesthetic. Instead, clean the foot well, apply antibiotic ointment, and if you can, connect with a veterinarian to start her on antibiotics. Many of these infections can get deep into the joints, and antibiotics will help speed up her rate of recovery. Separate her into a pen with deep, clean bedding material to limit the pressure on the foot, and to reduce the potential for more bacteria to enter the affected area.  If you can't get in contact with a veterinarian, you can still do the other at home treatments and let her heal on her own.

Mild soap and water to clean it will be fine. The main goal is to get rid of surface contamination before you put her in the isolation area with clean bedding. Any antibiotic ointment you apply will likely come off, but it can't hurt. Your vet will likely provide oral antibiotics and based on the bacteria they determine it is, may give you a broad spectrum or a more specific one. As of June 11th, 2023, all antibiotics now require a veterinary prescription. 

There are very few avian only veterinarians. That said, many exotic animal vets will see chickens, and some companion animal vets will as well. You will have to do some legwork and call to see who'll see her. Finding a vet is unfortunately a common struggle for most chicken owners. As I shared before, you can clean her up, keep her isolated on clean bedding, and she'll likely overcome the infection on her own in that reduced stress environment. There is no reason to cull her unless she's severely impacted by the condition, or you feel that she's suffering immensely. Best of luck!