Hi, I can understand your concern for smell from sebaceous cyst on neck. It appears that the area got secondarily infected. You should meet your treating doctor or preferably a surgeon to get proper diagnosis. Do not try to even touch or pinch the area as can cause more aggravation of the problem and spread of the infection. This area is quite common to get infected very easily because of sweating and high bacterial proliferation. I suggest you to get it excised at the earliest. For time being period you can apply antibiotic cream such as fucidin or mupirocin and take pain killer tablets such diclofenac or paracetamol. Take care.