Soft is encouraging. A tumor is hard. A tumor also has a blood supply while a cyst does not.
Here's what you need to do unless you would rather take the chicken to a vet. Assemble some hot water, small clean rags, some triple antibiotic ointment, and a sterilized sharp Xacto knife or razor blade.
Apply hot compresses, hot but not so it burns. Keep changing out the hot compresses for ten minutes. Then take the cutting blade and make a quarter inch slit in the middle of the cyst. If it bleeds, it's a tumor. Stop right there. If it doesn't bleed, it's a cyst. Apply pressure around the base and push out the pus. Keep it up until all of the waxy stuff inside comes out and nothing is left but pink watery fluid. If it's a blister, then push out all the fluid.
Clean the wound thoroughly with an antiseptic like Betadine, dry and apply the ointment and bandage if you like. Or keep her in a clean crate until the wound closes and heals over.