What exactly are you calling the "steering tube"? I assume you are talking about the quill stem (the silver aluminum piece that holds your handlebars). There should be a bolt on top, located at the back. You need to loosen that bolt and unthread it a bit... if it's stuck, tap the bolt heap with a rubber mallet. Then you should be able to remove the quill stem. If it does not come out, it may be rusted in place... which is no fun at all.
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