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')
}
}
})
'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.