How should I care for behind ear incisions after face and neck lift? They feel itchy and irritated.

Facelift incision care differs among surgeons. If this itching is tremendously bothersome then ask your surgeon to recommend or prescribe something to combat this. 

Consult your surgeon first but I think you should definitely stop the peroxide and topical antibiotic. It's likely the cause of the problems.  It's best if you don't put anything on it. I've seen plenty of patients run into this problem when treating the healing incisions with peroxide and topical antibiotic. I never have my patients apply peroxide and I only use topical antibiotic for a few days max.