Hi Shadan - starting with the URL, use "pin" instead of "password" to change the VM password:
PUT
https://<server>/vmrest/users/<userobjectid>/credential/pin As for why the changes don't work, there was a problem with this in UCxn 7.1.3 that was fixed in an ES, so if you are on 7.1.3 make sure you are on the latest ES so you have the fix for that.
~ Matt