You are able to change the target of your application to a different platform as long as it has the status "Unpaid". Once the application gets "Paid" status, you are not able to change its target, unless you duplicate it.
By clicking the drop-down menu for the "Target" field, you have the option to select iPhone, Android or iPad platform. Select the one you want your app to be created for.
Once you do so, the page will refresh and you will notice the online previewer will also adjust to accordingly to the selected platform.