How to modify a HashMap while iterating?

Question

I'm trying this (this is not a real code, just an example):

use std::collections::HashMap;

struct Foo {
    kids : HashMap<i8, i8>
}
impl Foo {
    pub fn update(&mut self) {
        let keys = self.kids.keys().clone();
        for k in keys {
            self.mul(*k);
        }
    }
    fn mul(&mut self, k: i8) {
        self.kids.insert(k, k * k);
    }
}

I'm getting:

error[E0502]: cannot borrow `*self` as mutable because it is also borrowed as immutable
  --> src/lib.rs:10:13
   |
8  |         let keys = self.kids.keys().clone();
   |                    ---------------- immutable borrow occurs here
9  |         for k in keys {
   |                  ---- immutable borrow later used here
10 |             self.mul(*k);
   |             ^^^^^^^^^^^^ mutable borrow occurs here

I think I understand why Rust complains, but what is the workaround? I need to get keys first and then modify the HashMap, going through it key by key.

I want to keep my mul() function, since in real code I do many more modifications there.