How to replace a character at random in a string?

I've printed a string of "+" symbols based on two given values(N, M). Now I'm trying to figure out how to replace characters at random in said string based on a third given value(K). The characters are stored in a string(l). I think I have to use the replace function but I don't know how(hence why it's in a comment for now). Any help is appreciated.

#include <stdio.h>

unsigned int randaux()
{
  static long seed=1;
  return(((seed = seed * 214013L + 2531011L) >> 16) & 0x7fff);
}

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

int main() {
    char s[1000];
    int N, M, K, l;

    printf("N: ");
    scanf("%d",&N);
    printf("M: ");
    scanf("%d",&M);
    printf("K: ");
    scanf("%d",&K);
    printf("\n");

    gets(s);

    l=strlen(s);

    /* Mostre um tabuleiro de N linhas e M colunas */

    if(N*M<K){
    printf("Not enough room.");
    }else if(N>40){
    printf("Min size 1, max size 40.");
    }else if(M>40){
    printf("Min size 1, max size 40.");
    }else{      
    for(int i=0; i<N; i++)
    {

    for(int j=0; j<M; j++)
    {
    printf("+", s[j]);
    }   

    printf("\n", s[i]);
    }
    for(int l=0; l<K; l++)
    {
    /*s.replace();*/
    }
}
    return 0;
}