Update installment plan

curl -X PATCH "https://edushade.com/api/v1/billing/installment-plan" \
  -H "Content-Type: application/json" \
  -d '{
    "down_payment_percent": 30,
    "fee_amount_in_cents": 200,
    "fee_type": "percentage",
    "installment_interval_days": 30,
    "interest_rate": 5.5,
    "interest_type": "flat",
    "num_installments": 6
  }'