Microsoft Skype for Business for Windows (last updated 10 April 2019)
Use these files to install the client - previous versions of the install software is no longer active. You don't have to reinstall the client on machines already running a previous version, however - these will still work.
If you don't know whether your computer runs 32 or 64 bit operating system, you can click the Windows button, search for System and click System in the section Control Panel.
Type will be listed in the section System.
Once you have installed the client, you will need to update it. The latest update can be found here - use the link that fits your installed version in the column Download. https://blogs.technet.microsoft.com/uclobby/2016/07/29/skype-for-business-2016-update-list/
Note that the Skype client is not updated by Windows Update. You will need to manually update it from time to time. You do that by using the above link.
Microsoft Skype for Business for Mac