AFAIK there is no clear method for priority_queue in C++. What you can do is naively pop elements out or create a new priority_queue:

pq = priority_queue<int>();

Now comeback to your question, I haven't seen anyway to change the comparator. However there is a simple workaround using custom comparator.

int sign = -1;
bool cmp(const int &a, const int &b){
    return a * sign < b * sign;
}

signed main(){
    ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);

    priority_queue<int, vector<int>, decltype(&cmp)> q(cmp);
    q.push(1); q.push(2); q.push(3);
    cout << q.top() << '\n';

    sign *= -1;
    q = priority_queue<int, vector<int>, decltype(&cmp)>(cmp);
    q.push(1); q.push(2); q.push(3);
    cout << q.top();
}

Output:

1
3

With sign equal to -1, small numbers will be prioritized and similar for sign equal to 1

It is possible but you will have to maintain the priority_queue manually using make_heap, push_heap, and pop_heap.

https://ideone.com/jBJc9w

To change the comparator of the std::proiority_queue, you may use a type of struct/class called functor. It is a struct/class which has a function operator().

For example, to use a heap that its top element is the smallest one. You can write as follows.

#include<bits/stdc++.h>
using namespace std;
struct myfunctor{
	int operator()(int x,int y){
		return x>y; // you can change it to what you want 
	}
};
priority_queue<int,vector<int>,myfunctor>q;
int main(){
	q.push(1);q.push(2);q.push(3);
	cout<<q.top(); // will output 1
}

There is already functors in STL called greater<T> and less<T>. Code above can also write as follows:

...
priority_queue<int,vector<int>,greater<int> >q;
...

If you found mistakes in my code, please comment me.

And forgive me for my poor English.