Luckily, inside of rules() method we have access to check HTTP method that has been used. As we know, store() comes with POST method, and update() uses PUT method.

so we can use something like this,

public function rules()
{
    $rules = [
        'name' => 'required',
        'email' => 'required',
    ];

    if ($this->getMethod() == 'POST') {
        $rules  = ['password' => 'required|min:6'];
    }

    return $rules;
}

And then, we can reuse the same FormRequest class in both methods, in the Controller.

public function store(StoreUserRequest $request)
{
    // ...
}

public function update(StoreUserRequest $request, User $user)
{
    // ...
}

I hope you got an idea.