In Site Manager -> Settings -> <global settings> -> Security & Membership -> Administration section there is setting named “Use SSL for administration interface”. Please note: Before setting this key to true, please make sure that the SSL certificate is correctly installed in your IIS. If not, you won’t be able to login to the UI, and you will need to change this setting back to false directly in the database e.g. using this query:
UPDATE CMS_SettingsKey
SET KeyValue = 'false'
WHERE KeyName = 'CMSUseSSLForAdministrationInterface'