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You have both the DD tail motor and prop - right.
Just cut the old prop off the DD tail shaft, using either a dremel tool or sharp craft knife - and press on the new one.
If you have the stock geared tail, you need to replace that with the DD motor and prop, not just the prop.
Also, for future reference goto the Century Hummingbird section of Runryder...a Dragonfly is basically the same as a Hummingbird V2.
You will learn loads about the heli in this section, and you will get quicker responses to your questions.