In the ResetPasswordController there is a trait which includes a lot of the code, and the validation rules.
In a class, you can override the trait by including the same name method, so if you copy the code from the trait to your class you can amend it how you need.
Create a method (in Auth/ResetPasswordController.php) as below;
/**
* Get the password reset validation rules.
*
* @return array
*/
protected function rules()
{
return [
'token' => 'required',
'email' => 'required|email',
'password' => 'required|confirmed|min:6',
];
}
And change the rule as you require