public OnPluginStart()
{
     new String:text[] = "abcdefg";
     PrintToServer(text);

     ChangeCharByIndex(text, sizeof(text), 2, '\0');
     PrintToServer(text);

     text = "abcdefg";
     ChangeCharByIndex(text, sizeof(text), 2, 'X');
     PrintToServer(text);
}

stock ChangeCharByIndex(String:text[], text_size, index, new_char)
{
     decl String:buffer[text_size]; buffer[0] = '\0';
     for (new i = 0; i < index; i++)
     {
         if (buffer[0] == '\0') Format(buffer, text_size, "%c", text[i]);
         else Format(buffer, text_size, "%s%c", buffer, text[i]);
     }
     if (new_char == '\0') Format(text, text_size, "%s%s", buffer, text[index+1]);
     else Format(text, text_size, "%s%c%s", buffer, new_char, text[index+1]);
}

abcdefg
abdefg
abXdefg