How do I set a previous base SDK in Xcode 10?

BTW, you can see or select the Base SDK you are using, in the Xcode Preferences/Locations:

The base SDK is just "iOS". The exact version will depend on your version of Xcode (12.1 if you are using Xcode 10.1). The base SDK does not determine which versions of iOS are supported by your app.

You set the minimum version your app is compatible with by specifying the "iOS deployment target", either for your project as a whole:

or in the settings for a specific target in your project

The oldest version of iOS that you can target with Xcode 10 is iOS 8 (which is pretty old).