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I think your best bet would be to trim the hair around the area a much as possilbe to cut down on bacteria. You can then wash the area with an antibacterial soap, disinfect with hydrogen peroxide and apply an antibiotic ointment or neosporin to the area. You can cover it loosely with guaze to help prevent further damage. In addition, you might want to apply some warm compresses to the area a few times a day as well. Applying a sock to the paw being used to scratch the area may also cut down on additional damage to the lump as well.
She should be seen by your vet though when possible to check for infection, growth and reevaluate possible surgical removal. I hope this information is helpful to you.