HOW TO GET IT:
• If your console is in Instant-On mode and you have been selected to receive the update, it will download and install the update and reboot to standby.
• NOTE: Once the update is available to you, you can manually begin the update by launching the Guide and navigating to All Settings > System > Updates.
• If your console hasn’t downloaded and installed the update by 1:00 AM PST 12/6/2017 (9:00AM GMT 6/12), you will be prompted to do so at that time.
• NOTE: This update has a shorter than usual amount of time before it becomes mandatory.

DETAILS:
OS version released: rs3_release_xbox_dev_1711.171203-1700
Available: 2:00PM PST 12/5 (10:00PM GMT 5/12)

FIXES:

• Networking
• Fixed an issue which caused certain wireless networks to be missing from the list when scanning for available wireless networks.
• Fixed an issue which sometimes caused unexpected network disconnections when using a wireless connection.

• Games/apps not installing/updating/launching
• We investigated reports of some users in ASIA/APAC(Australia & Pacific) countries that could note install/update/launch any games or apps. We have conduced service changes in the global network for ASIA and APAC countries that should resolve these errors.

KNOWN ISSUES:

• Display
• We are investigating the inaccurate RGB colors that have been reported when displaying in 4K HDR mode when playing a UHD disc.
• Tournaments
• The left and right navigation for selecting date and time during Tournament creation is currently reversed when the console language is set to Arabic or Hebrew.
• Left and right navigation in Tournament twists is reversed when the console language is set to Arabic or Hebrew.
• Left and right navigation in the bracket view of Tournaments is reversed when the console language is set to Arabic or Hebrew
• Avatars on Home
• Users wishing to represent themselves as an avatar can do so by changing their settings under My profile > Customize profile > Show my avatar.
• Narrator
• Narrator can stop reading large amounts of text.


Source: December 5th: New Preview DELTA & OMEGA Beta build available (1711.171203-1700) | Xbox Forums