You’d have to go through support. Not sure if they’ll refund you but you can probably get an RMA if it is a hardware failure. many times its not a hardware issue though. something to keep in mind. 

https://www.surfboard.com/support/resources/

I tried but there is no way to find a place to return an item, My problem is that the g36 doesn’t work with xfinity, i have to reset it every two days, and no is not a problem with the wireless as one moderator answer to another that have the same problem, i’m connected to the G36 wired. I bought a motorola modem and i have no problem now, but as ARRIS don’t even took the chance to answer or have a technical support technician available my evaluation and feedback about them will be and is the worst, I lost 340.00 USD when i bought this G36 modem that still have more than 10 months of warranty

I’m in the same boat that you are.  My g36 continuously says that it’s offline with Xfinity every 1-2 days and I have to reboot it.  It’s still within the year warranty and I don’t want a replacement because this modem has so many complaints about it with Xfinity.  They are expensive and unfortunately I just had to drop almost $500 to replace it with another brand.