According to Samsung, there have been 35 reported cases of defective Galaxy Note 7 batteries. In response, they will voluntarily replace all existing Galaxy Note 7 devices “over the coming weeks”. They have also stopped selling the devices, presumably, because they are capable of fixing the devices for existing customers, until the stock can be replaced.
This comes after reports that Galaxy Note 7 phones have been either catching fire or exploding. Some outlets are claiming that Samsung has confirmed 35 cases of fire or explosion, but, unless these outlets have more information than on the public statement, Samsung has only confirmed 35 complaints, and it's possible that other, related issues were included in that tally (like feeling excessively hot).
They did not mention a specific way for Galaxy Note 7 owners to request a replacement in their press release, but their technical support contact information is available here. I assume that they will point you in the right direction.