class Stripe::Coupon

def self.update(id, params = {}, opts = {})

Updates the metadata of a coupon. Other coupon details (currency, duration, amount_off) are, by design, not editable. 
def self.update(id, params = {}, opts = {})
  request_stripe_object(
    method: :post,
    path: format("/v1/coupons/%<id>s", { id: CGI.escape(id) }),
    params: params,
    opts: opts
  )
end