If you are trying to find the most current Mitel MiCollab Client (formerly Unified Communicator Advanced UCA) on the internet, you are looking in the wrong place. You already have it. It is stored on your Mitel MAS server.
To download the most current Mitel UCA build your hardware can handle just use this URL:
http://<your micollab server name or IP address>/ucs/dl/UnifiedCommunicatorAdvanced.msi
This took me quite some time to locate but in my case, UCA 6.0.422.0 worked just fine on Windows 10 Pro 64 bit AFTER I made a registry change to avoid the dreaded MiCollab CONNECTION LOST.
Note that the big change from v5 to v6 UCA is that it no longer requires the desktop to request a certificate from the MAS / MGB server.