template<class ForwardIterator, class T>
constexpr void
replace(ForwardIterator first, ForwardIterator last,
        const T& old_value, const T& new_value);

template<class ExecutionPolicy, class ForwardIterator, class T>
void replace(ExecutionPolicy&& exec, orwardIterator first,
             ForwardIterator last, const T& old_value, const T& new_value);

#include <iostream>
#include <algorithm>
#include <string>

int main()
{
    using namespace std;
    using namespace std::string_literals;

    auto s = "hello world!"s;

    replace(std::begin(s), std::end(s), 'h', 'H');
    std::cout << s << std::endl;
    return 0;
}

Hello world!