The problem one is going from string format in one way to a string format in another.

If(hh >=12 )
    {
        suffix = "PM";
     hh=   (hh>12) ? hh-12:hh;
    }
    else
    {
        suffix = "AM";
    }

If high performance is an issue, then you would want to improve on this.

#include 
#include 
#include 
#include 
#include 

string change2(string time24Hour)
{
   char buf[3];
   int hh = atoi(time24Hour.c_str());
   string suffix = hh > 11 ? "PM" : "AM";
   hh = hh > 12 ? hh - 12 : hh;
   sprintf(buf, "%02d", hh);

   return string(buf) + time24Hour.substr(2, string::npos) +  " " + suffix;
}

Regardless of the semantics of what one calls “pure” C++ code, the proposed answer produces “00 AM” and “00 PM” instead of “12 AM” and “12 PM”
This can be corrected with

        oss << std::setw(2) << std::right << std::setfill('0') << (hour+11)%12+1;
        time24Hour.replace(0,2, oss.str());
        time24Hour += hour>=12?" PM":" AM";