Help with upgrading app to .NET 4.0

I have been writing and publishing .NET apps using .NET 3.5, but I now want to use 4.0 so I can draw Charts.
I have my app working and it runs in Visual Studio in the debugger.
When I publish and try to access the app from a Client I first got:
Unrecognized attribute 'targetFramework'. Note that attribute names are case-sensitive.
Line 32:     </httpHandlers> Line 33:     <customErrors mode="Off"/> Line 34:     <compilation strict="false" explicit="true" targetFramework="4.0"/> Line 35:     <pages controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID"> Line 36:       <controls>
So I went into IIS and Properties, for my app ...