How can I do the same ?
Packet data follow the packet header.
If I change the data and recalculate the chksum; what special care does the code have to take ?
If you don’t modify packet length then checksums (IP/TCP/UDP) recalculation is enough. If you do modify packet length (an example change “sex” to “fofo”) for TCP protocol you also have to care about Sequence and Acknowledgement numbers.