What's the best treatment to get rid of familial cold urticaria?

Stay warm. Move to a warmer climate. Don't take cold showers or baths. Never swim in cold swimming pools or in cold ocean water. Dress warmly including gloves and scarves in cold weather even if you're going outside for just a minute or two to take out the trash. Prophylactic antihistamines - that means taking it everyday may help. There is no cure for physical urticarias like cold urticaria.
Treat as needed. Familial cold urticaria cannot be cured but it can be treated with allergy meds and minimizing environmental conditions that trigger it like cold etc.