Function which replaces every digit inside of a string with a random digit

First of all I want to thank everyone who has helped me on my last issue, it helped me a lot with understanding of how a C program runs in the background and what loops are used for what. Now I'm encountered with this task: write a function which replaces every digit in a given string (0, 1, 2,... 9) (ASCII values: 48, 49, 50,... 57) with another random digit. Every other type of data in a string needs to remain untact. The task also says that, in main(), I'm supposed to create a string capable of holding 50 elements and do dynamic memory allocation for 50 char type elements - full capability of memory handling, and in the end print out the result of the function.

This is what I've written so far:

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

char* switching(char s[]){

    int i;
    srand((unsigned)time(NULL));

    for(i=0; i<strlen(s); i++){
    
        if(s[i] > 57 || s[i] < 48){
        
            s[i] = (float)rand()/RAND_MAX * 9;
        }
    }
    return s;
}

int main(){

    char *string = (char *)malloc(50);

    if(string == NULL){
        return 1;
    }

    char *r;

    r = switching(string);
    printf("%c" , r);
    free(r);

    return 0;
}

And this is what my apparent error is (although there might be more of them but I'm not sure) when i run it in OnlineGDB:

               main.c:32:18: warning: format ‘%c’ expects argument of type ‘int’, but argument 2 has type ‘char *’ [-Wformat=]     
                                                                                                                
                                                                                                                
        ...Program finished with exit code 0                                                                                
        Press ENTER to exit console.

So yeah, as last time, any help would be appreciated. Thanks in advance to anyone that tries to help.