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If you're refering to launchers (home screen), you can go to "Settings / Applications/ Manage applications", look for this unwanted launcher (probably it will be called "Launcher", I don't know the exact name, don't have Android launcher on my phone), click on it, go to it's section "Launch by default" and click on "Clear defaults". Next time you press home button or go back to hemescreen, system lets you choose launcher again.