Using V6.0 on Older version in VS 2017

I have recently upgraded to VS 2017.

I built a replica version of one of my Production websites which was built in VS 2013 targeting framework version 3.5.

When creating the project in VS 2017, I set the target framework version to 3.5 on the very first page.

I am now hoping to utilise some of the C# language features from C# 6.0.

There doesn't seem to be a way thru property pages or other to set the version of C#.

I know there is an advanced button on the properties page if the project is targeted to 4.6.x on creation, but that button doesn't exist in VS 2017 when targeting 3.5 on creation.