If you live in the US and your Axon 7 features the model number of A2017U, registering for the preview of Android Nougat is fairly straightforward. If you don't have one already, you'll need to create an account for the ZTE US Community. Following the creation of your account, all you have to do is click on the source link below to sign up with the official registration forum. Once accepted, you'll be able to download Android 7.0 Nougat on you Axon 7 before the official release goes out to the mass public.
Although ZTE hasn't announced official plans for the public release of 7.0 Nougat, this preview program suggests that we'll likely see the update be available for everyone either at the end of this month or at some point in early January. Either way, if you're alright with putting up with a couple of bugs that are likely to exist, you can register to start testing out 7.0 Nougat on your Axon 7 today.