replace string in byte stream

I have a function that read data byte and drmp to a file

how can I change the string <givenname xsi:nil='true'/> to 
<givenname>.</givenname>

======================
bool dump;
std::ofstream dumpfile;


int my_block_reader(void *userdata, const char *buf, size_t len)
{
	if (dump == false) {
		if (strnicmp(buf, "<?xml", 5) == 0) {
			dump = true;
		}
	} 
	if (dump == true) {
		char *szBuffer = new char[len + 1];
		memcpy(szBuffer, buf, len);
		szBuffer[len] = 0;
		dumpfile << szBuffer;
		delete[] szBuffer;
	}
	return 0;
}