There are two ways to extend ISP and Datacenter Proxies orders: as a subscription and manually. In both cases, you can select to keep specific IPs in your current order.
For manual extensions:
Select 'ISP/Datacenter' from the sidebar menu
Click the 'Options' button next to your active order and select 'Extend'
Click the 'Extend your chosen proxies' section
Choose the IPs you want to keep and click 'Continue'
For subscriptions:
Select 'ISP/Datacenter' from the sidebar menu
Click the 'Order details' tab
Click the 'Extend your chosen proxies' section
Choose the IPs you want to keep and click 'Continue'