EFN

It was 10 degrees when I turned my car on, I got a Volt Workshop message on my display, I would like to replace the battery, the message has never come on again, should I replace both batteries or just the main, do I need and OEM for $265, where can I find the security code for the radio, and what is this about re-syncing windows, esp, sunroof, I have the original paperwork, is the security code the 5 digit number inside the leather case I got with the car,

Can I do this myself, anything I should know??