Smart Key Remote Problem

Hello. I have a 2011 Sentra SR. I bought it new and it came with 2 smart key units. The main one I have been using since new, just died. The other one works perfectly.

My question is: is there a battery in there? Is it easy to change out? How do you change it out? What kind of battery do I need? And where can I get one? Estimated price?

Final question? I am still under the 3/36 bumper to bumper warranty...is this covered by the warranty?

Thanks so much for your help.
Sincerely,
Chris