Hire a professional and then hire them each year to winterize system. If you still want to do it yourself then loosen unions. Remove cracked portion and replace. Repeat steps in reverse order to re-install
If you are determined to do it your self, I suggest taking pictures and showing them to the employee at the parts counter so he can get you the proper supplies. Your best bet is to take the previous advice since you are required to have the backflow tested after work is preformed on it or it is replaced.
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