Download macOS High Sierra 10.13.6 Update

The macOS High Sierra 10.13.6 update adds AirPlay 2 multi-room audio support for iTunes and improves the stability and security of your Mac. This update is recommended for all users.

AirPlay 2 for iTunes
Control your home audio system and AirPlay 2-enabled speakers throughout your house
Play music at the same time on multiple AirPlay 2-enabled speakers in your house, all in sync

Other improvements and fixes
Fixes an issue that may prevent Photos from recognizing AVCHD media from some cameras
Fixes an issue that may prevent Mail users from moving a message from Gmail to another account