At the time of writing (18/07/2017) the latest PowerShell available is PowerShell 5.1 which is a part of Windows 10 1607 and Windows Server 2016.
To find which version of PowerShell you have, type this command into PowerShell:
To upgrade PowerShell, you’ll need the latest Windows Remote Management for your version of Windows. You might also need to upgrade WinRM to remotely manage servers using the Server Manager tool, which comes with Windows Server and is a part of RSAT for Windows 10 version 1607 and later.
At the time of writing the latest version is 5.1 and it is available to download for Windows 7 Service Pack 1, Windows 8.1, Windows Server 2008 R2, Windows Server 2012, Windows Server 2012 R2.
You can download Windows Remote Management 5.1 here.
In order to install WinRM 5.1 you’ll also need Microsoft .NET Framework 4.5 or above. At the time of writing the latest version is 4.7.
You can download the Microsoft .NET Framework 4.7 Offline installer here.
Hope this helps make some sense of the upgrade path.