Just follow the simple steps:

1. Assume you have a domain called "example.com".

2. Jus go to Home > DNS functions > Edit DNS zone

3. Choose the example.com and edit.

4. There, after the 2 entries of nameservers, you will have A record pointing to the current Server IP.

5. Change the IP of Which server you need to point out.

6. At last you may reload the DNS service.