struct node
 {
     int label;
     long cost=LONG_MAX;
     bool visited=false;
     bool operator < (const node &other) const { return cost < other.cost; }
 }

set<node> S;

//here i just want to update the cost of a node in my set S

set<node>::iterator it=S.find(vertex);
*it.cost=200;

auto it = S.find(vertex);
node cur = std::move(*it);
auto next = S.erase(it);
cur.cost = 200;
S.insert(next, cur);

auto node = S.extract(vertex);
node.value().cost = 200;
S.insert(std::move(node));