template<class ForwardIterator, class Predicate, class T>
constexpr void
replace_if(ForwardIterator first, ForwardIterator last,
           Predicate pred, const T& new_value);

template<class ExecutionPolicy, class ForwardIterator, class Predicate, class T>
void replace_if(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last,
                Predicate pred, 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_if(std::begin(s), std::end(s),
               [](auto ch){
        return ch == 'h';
    } , 'H');
    std::cout << s << std::endl;
    return 0;
}

Hello world!