In Plesk 12.5 and below there was a button Change Hosting Settings:
Where can this button be found in Plesk Onyx?
According to Plesk Onyx release notes, the Change Hosting Settings screen for a subscription was eliminated as its features are available on other screens.
Web Hosting Access option allows to change subscription's IP address and system user's name/password.
Default database server can be chosen at Tools & Settings > Database Servers