how to replace the most common character in a string with the second most common in the string in c?

I wrote a C function that receives a string from the user, and replaces the letter with the highest frequency, with the letter with the second highest frequency in the string. Example: For the input i love you more, the string i levo yeu mero will be returned

#include <stdio.h>
#include <string.h>

void stringReplace(char str1[]);

int main()
{
    char str1[100] = { 0 };
    stringReplace(str1);
    return 0;
}

void stringReplace(char str1[])
{
    char ch1, ch2;
    int i, h, j, p, n, len, counter1 = 0, counter2 = 0, first, second, times;
    printf("Please enter the string - maximum = 100 characters:\n");
    printf("User input: ");
    fgets(str1, 100, stdin);
    str1[strcspn(str1, "\n")] = 0;
    len = strlen(str1);
    for (i = 0; i < len; i++) {
        counter1 = 0;
        for (j = 0; j < len; j++) {
            if (str1[i] == str1[j]) {
                counter1++;
            }
            if (counter1 > counter2) {
                first = i;
            }
        }
        counter2 = counter1;
    } //character which shows up most - found.

    counter2 = 0;

    for (p = 0; p < len; p++) {
        for (n = 0; n < len; n++) {
            if (str1[p] == str1[n]) {
                counter1++;
            }
            if (counter1 < first && counter1 > counter2) {
                second = p;
            }
        }
        counter2 = counter1;
    }

    ch1 = str1[first];
    ch2 = str1[second];
    
    for (h = 0; h < len; h++) {
        if (str1[h] == ch1) {
            str1[h] = ch2;
        }
    }
    puts(str1);
}

This is the code I have done, and instead of changing the string it prints the same thing. for example:

Please enter the string - maximum = 100 characters:
User input: i love you more
i love you more

Suggest : divide and conquer with helper functions.

Think about the sub-problem: Counting the 1st & 2nd most popular.

Walk the string. At each character count its occurrence with the rest of the string. O(n*n)

When a better than 2nd place count is detected, adjust 1st and 2nd place.

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>

void most_popular2(const char *s, char pop[2]) {
  pop[0] = pop[1] = 0;
  size_t pop_count[2] = {0, 0};

  while (*s) {
    // Do not count white-space.
    if (!isspace(*(const unsigned char* )s)) {
      // How popular is *s?
      size_t count = 1;
      // sum all the matches on the rest of the string
      for (const char *r = s + 1; *r; r++) {
        if (*s == *r) {
          count++;
        }
      }

      // Test if better than the 2nd most popular.
      if (count > pop_count[1] && *s != pop[0]) {
        if (count > pop_count[0]) {
          // Demote 1st place to 2nd place.
          pop_count[1] = pop_count[0];
          pop[1] = pop[0];
          // Save new 1st place.
          pop_count[0] = count;
          pop[0] = *s;
        } else {
          // Save new 2nd place.
          pop_count[1] = count;
          pop[1] = *s;
        }

      }
    }
    s++;
  }
}

Sample

int main() {
  char s[] = "i love you more i love you more\n";
  char pop[2];
  most_popular2(s, pop);
  printf("1st:%c 2nd:%c\n", pop[0], pop[1]);
}

Output

1st:o 2nd:e

Leave it to OP to take the 2 most popular characters and form the desired output.

More efficient code could copy the string, sort it O(n*(lng(n)) and then walk it to find the 2 most popular.