Platform Target Framework Version greater than Project Target Framework Version.

I'm getting this error using Xamarin Forms in Visual Studio 17: The $(TargetFrameworkVersion) for Xamarin.Forms.Platform.dll (vv8.0) is greater than the $(TargetFrameworkVersion) for your project (v7.1). You need to increase the $(TargetFrameworkVersion) for your project.

I need to know how to change the Xamarin.Forms.Platform.dll to be compatible with my Target Framework Version, which is currently 7.1, but I'd like to get down to 6.0 and 5.1 as well. I'm also getting the same error for two other .dll files as well. I'd appreciate some help with this.

Thanks,
-Curtis