If your device is used in the S mode converter, you have a set of S mode functions that are available and lets you change the primary address. The function that should be used in this case is caddr [addr]<CR>
is the argument that specifies the new GPIB address for the GPIB-RS232. The argument consists of a primary address and an optional secondary address. The secondary address is separated from the primary address by a plus sign (+). Both addresses are expressed as numeric strings.
If you specify a primary address without a secondary address, secondary addressing is disabled.
If you call caddr without an argument, the GPIB-RS232/485/422 returns its current GPIB address.
This is an example on how to call the function:
char buffer  = "caddr 0+22\r";
viWrite(instr, buffer, strlen(buffer), &retCount);
The address can also be changed using NI MAX as shown in the article Changing a GPIB Interface in NI MAX.