Upgrade error

What do I need to do to address this?
Trying to upgrade to 2.7.14 from 2.6.4

W: There is no public key available for the following key IDs:
9D6D8F6BC857C906

Maybe I can get some help on this ongoing project.
I have extremely limited knowledge in doing the following, step by step instruction are desperately needed.
I tried to "Upgrade" 2.6.4 to 2.7, I'm having all kinds of issues with repositories and such.
I decided maybe I should just make a new install USB to follow along with progress, that's turning into some kind of adventure.

I used "zsync" in a terminal and have LCNC 2.7.14 downloaded, all I need to do now is get it on the USB, I ran "dmesg" to try and figure out were the USB is, can someone share the code to install the download to the USB with the out I have here?

[186785.145828] usb 2-1: Product: USB Flash Drive
[186785.145834] usb 2-1: Manufacturer: Lexar
[186785.145841] usb 2-1: SerialNumber: AAMS5FD84X0BSULW
[186785.147212] usb-storage 2-1:1.0: USB Mass Storage device detected
[186785.149926] scsi host7: usb-storage 2-1:1.0
[186786.322462] scsi 7:0:0:0: Direct-Access Lexar USB Flash Drive 1100 PQ: 0 ANSI: 6

Thanks in advance.

Never mind I sorted this.