Quick method to determine installed version of .NET Framework
We use WSUS to update our servers and I blocked the install of the latest .NET patches because of the issues with Exchange. The Lync team is also recommending not to install these .NET updates also because of the close integration of Lync and Exchange.
Due to recent issues with unsupported versions of .NET being installed on Exchange servers, as well as the fact that Exchange Server requires specific versions of .NET to be installed (Exchange Server 2013 System Requirements & Exchange Server 2016 System Requirements), there is a need to quickly query the installed version of .NET on Exchange servers. I have also been involved in several Exchange support escalations where updating the Exchange servers from .NET 4.5.1 to 4.5.2 resolved CPU performance issues.
PowerShell Query Method
To query the local Registry using PowerShell, execute the below command in an elevated PowerShell session.
(Get-ItemProperty ‘HKLM:\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full’ -Name Release).Release
You can then use the table below to reference the installed version of .NET. For instance, if the returned value is 379893, then .NET 4.5.2 is installed.
Source: Quick method to determine installed version of .NET Framework | Troubleshooting Exchange