How to change the password policy?

There's no Admin UI for changing the password policy. You'll need to navigate to the XWiki.Registration page and edit it. Then find and modify the following block:

#set($field = {
  'name' : 'register_password',
  'label' : $services.localization.render('core.register.password'),
  'params' : {
    'type' : 'password',
    'autocomplete' : 'off',
    'size' : '60'
  },
  'validate' : {
    'mandatory' : {
      'failureMessage' : $services.localization.render('core.validation.required.message')
    },
    'regex' : {
      'pattern' : '/.{6,}/',
      'failureMessage' : $services.localization.render('xe.admin.registration.passwordTooShort')
    }
  }
})

Specifically you may want to modify the regex pattern.