When you buy a subscription for the first time, it will renew automatically.
If you want to turn on the subscription for any of your active orders:
Find Datacenter proxies in the dashboard
Click on the order (1) you need
Click on Order details (2) tab
Enable the Subscription (3) and configure your plan (4)
Choose the payment method (5) and hit Save (6).
The system will automatically extend your order before it expires, so you’ll keep the same IP addresses. A 1-day order will automatically extend by another 24 hours, a 30-day order will automatically extend by another 30 days, and so on. The system will automatically include all discounts you’re entitled to.