Backflow preventer cracked.

That isn't a backflow preventer (aka "check valve") it is a union. Only way to fix that is to replace the piece on the right which means cutting back the plumbing until you have enough room to add a patch piece. This also assumes you can find the same model union as they are usually proprietary. But that looks like the usual union sold at Home Depot.
 
Also, am I right to assume that i can just unscrew the coupling and just put a new one on?

Yes as long as it is the same one. But that does look like the standard Home Depot model. Not really a huge deal since it looks like you have a threaded fitting to the left side that you could replace up to the threads if needed.
 
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