Running an older iOS version in the iPhone simulator

I want to test my application on older iOS versions. I am using weak linking for new frameworks and branches in the code for different OS version. But after some complaints from users running iOS 3.1.2, I need to do some testing myself. I don't have a separate iPhone for every iOS versio开发者_如何转开发n.

So, how can I run an older version of iOS in the iPhone simulator?

That's a really basic question and there might be a duplicate somewhere, but I wasn't able to find it. Any help is much appreciated.

If you can download (or have a backup of) the older SDK that relates to the older OS you want to run on, I believe the general advice is to install the other SDK to a different path (say, Developer/olderSDK and this way you can have both SDK's (and simulators) installed side by side.