As per your history, if the growth in not increasing in size and as the picture you have attached, it seems to be a benign lesion. It can be a case of epidermal cyst.
Rarely it can be a cancerous lump.
But the treatment is surgical excision only if you want to get rid of it. There's is no medicine to dissolve the swelling.
Next Steps
Kindly consult a General Surgeon for further management