You don't need the agent to help you. The procedure is as follows.
1. You need to go to immigration dept to collect the information and form regarding the renewal of contract. Then you know how to do it.
2. After filling the form you can submit it to immigration dept according to the guideline.
3. Attention: Your attention is drawn that your helper's VISA is only for come in and out HK for ONE time. If the helper go back to their original place and back to HK within the original contract period, you need to renew the VISA before they leave HK otherwise they cannot go back to HK.
4. Attention: The immigration dept will only accept renewal within one month before expiry of VISA. Early application would not be accept unless a strong reason.
5. For details, you can ask Immigration Dept.