You need to set the deployment target to any minimum version of iOS that you want to target. You can install this in your target program, as shown in the screenshot below:

However, it is recommended that you keep the base SDK in the latest released version; which is currently 5.1:

Please note that in order for users to now be able to download your application on the new minimum version, you will need to re-download the new binary in the App Store for viewing and acceptance.
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