If you want to use the domain name without interruption, toggle auto-renewal in the properties for the domain when enabling the service. Make sure that you have sufficient funds in your account at the time of renewal to enable the domain fees to be withdrawn automatically; this also ensures the valid state of the domain name for the following registration period.
You can also renew manually. In the BILLmanager, go to Products/Services → Domains, select the domain and click Renew.