You'll have to apply for the privilege to be one of the first -- and fast, since it's first-come, first-served -- by making a Samsung account and then downloading the Galaxy Beta Program or the Samsung Members app. Since the Korean conglomerate introduced the program for Android Marshmallow last year, you might already have either app. In case you do get early access to Nougat, you'll obviously be able to preview Samsung's updated user interface and provide feedback on anything you think needs fixing.
Take note that that if you somehow managed to snag a limited edition S7 edge Olympic Games or an S7 edge Injustice Edition, you won't be able to participate in the beta. Further, you're only eligible in the UK if you have an unlocked device. In the US, on the other hand, you'll have to be a Sprint, T-Mobile or a Verizon subscriber. If you can't participate in the beta or end up not getting a slot, Sammobile has some screenshots of Nougat on a Galaxy S7 so you can get an idea of what it would look like on your device.