Modbus TCP - holding registers

Hello! I am working with a RCM5400W and I need to use the Modbus TCP communication protocol, so I am using the RCM5400W as the slave.

What I need to know is how can I access the Holding Registers? For example, to write the data that I need to send to the master or to read data the master sent. I need to know how to do this using Dynamic C.

Thank you