Change custom class value in range based for loop

Question

I'm building a data structure to keep track of count and rank of elements. I have a set of a struct I made and when I'm trying to modify the members in the struct with the range based for loop I get this error:

error: cannot assign to variable 'it' with const-qualified type 'const item &'

I already overloaded the < operator. What do I have to do more to make this work?

#include <iostream>
#include <stdio.h>
#include <set>
using namespace std;

struct item {
        int val, count;

        item(int v, int c) : val{v}, count{c} {}
};

bool operator<(const item &a, const item &b) {
            return a.val < b.val;
}


class DS {

    set<item> S;

    public:
    void notWorking(int x) {
            for (auto &it : S) {
                    if (it.val < x) it.count++;
            }   
    }   
};


int main() {                                                                                                                                                                                                                                

    return 0;
}