You have to enable this from the Editor itself. By the way, you can now download the latest version which is no longer in beta mode.
Go to Edit --> Project Settings --> Player --> Other Settings --> Configuration --> Scripting Runtime Version --> .NET 4.6 Equivalent
then
Go to Edit --> Project Settings --> Player --> Other Settings --> Configuration --> Api Compatibility Level --> .NET 4.6
This menu may have changed but this where it used to be and I expect it to be there or under that menu.
EDIT:
You must restart Unity Editor then restart Visual Studio to make these changes to take effect.
".NET 4.6 Equivalent" Menu has changed to "Experimental (.Net 4.6 Equivalent)".
If after restarting both Unity and Visual Studio but the version is still not changing, re-install both Unity and Visual Studio and make sure that you are running the latest version of Visual Studio.
Before re-installing visual studio - make sure you installed the latest update, if not try to update first.
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