As Denon is now loading the firmware updates on their website, having never been through the process myself, I wanted to do so so I could share my experience with you 2015 model owners. Note that the firmware update also comes with a set of .pdf instructions much of which are repeated below for clarity.
1. Go to http://usa.denon.com/us/downloads/productupdates
on your laptop or PC
2. Select the model of your AVR from the dropdown (note: it may take a week or more for update to be listed as a USB download after being released via the internet download)
3. Enter the serial number of your AVR (can be found on the OEM box as well as the back panel of the AVR)
4. Enter your personal info (name, address, email)
5. If download doesn't start within a few seconds, try again later that day or the next day (it took me a week of unsuccessful tries after the update first appeared on the webpage before I was able to successfully download the update for the X6200W)
6. Click on the downloaded zip file to unzip it (eg. AVR_6200W_USB_Update_02_18_16.zip) - there should be two items: (1) firmwares folder, and (2) Firmware Update instructions
7. Review the Firmware update instructions
8. Move the "firmwares" folder to a FAT16 or FAT32 formatted USB thumb drive (note: it's okay if there are other files on the thumbdrive)
9. With the AVR in Standby (all Zones off), place the USB thumb drive into the AVR's USB port
10. Press/hold the OPTION + STATUS buttons on the front panel (will be different depending on the AVR model) while powering on the AVR
11. Keep holding the OPTION + STATUS buttons until "USB Update Start" is displayed on the front panel (~ 10 seconds)
12. Press ENTER on the AVR's remote control and "Update File check" will display on the front panel (on the TV will display "Please wait ..... Checking update files"
13. It will take about 2 minutes before the update process starts, the front panel displaying the approximate time (in my case 28 minutes was displayed but actual time was 34 minutes)
14. When the update process is complete, "Update Complete" may display (I missed it) and the AVR will return to the last source selected.
15. Remove the thumb drive from the USB port and power cycle the AVR OFF, then ON again.
16. The Firmware update instructions .pdf shows a process to confirm the various updated file versions (all checked out for me)
After doing the update which completed successfully, there were two non DTS:X firmware update related issues that presented on my AVR:
(1) The INFO button no longer worked - I was able to resolve this by doing a "soft reset" (power to Standby and unplug the power cable for 10 minutes)
(2) When I tried to play Pandora, a "No network connection" message displayed. When I checked the settings, there was no router password so I tried several times to enter it, each time getting an "Incorrect password. Try again." message. Further checking of the Networking - Information settings showed there was no IP Address listed. I did several Network resets (see post 5 of this thread); however, could not get the network card to grab an IP address with DHCP set to ON. I was only able to regain network connectivity by setting DHCP to OFF and assigning a static IP address (see AVR Owner's manual for instructions).