I don't know your level in C/C++ coding.
What you need to do in a MITM (Man in the middle) attack.
When your computer is sending back the packet, you need to intercept them using a c++ packet sniffer library (a fast search over the internet give

).
Once you have intercepted the packet, get the hex of it, apply a regex or whatever search function, get the position of the bytes in the string to replace and inject yours. Send back the packets.