If you're a member of the Xbox One Preview Program, you probably consider yourself to be an early adopter. Because of that, you probably want to get your hands on the Windows 10 Anniversary Update (Redstone) for Xbox One.
As it turns out, it looks like you won't have to wait much longer. Xbox team member Mike Ybarra took to Twitter to say that the next Xbox One Preview will be released in late May to early June.
Note that while he mentions the Anniversary Update and the next Preview update in the same tweet, he doesn't specifically state that the update will be for the Anniversary Update; however, it is very likely, especially since Ybarra went on to confirm that there will be Preview updates between now and then.
Of course, if you simply can't wait a couple months, the new Dev Mode will get you build 14290, which is an Anniversary Update preview, even if it doesn't have any of the cool new features. Also, once the Anniversary Update is released, the Insider Program and the Xbox One Preview Program are going to be combined, so if you just can't manage to grab an invite for the Preview Program, you won't need one soon enough.