Via software update or directly through Apple: http://support.apple.com/kb/DL1103

File Size: 90.63 MB (Mac) 78.10 MB (Win 32) 78.98 MB (Win 64)

What's new in iTunes 10.1

Use AirPlay to instantly and wirelessly stream videos from iTunes to the all-new Apple TV.
Sync with your iPhone, iPad, or iPod touch with iOS 4.2.
Provides a number of important stability and performance improvements.
For information on the security content of this update, please visit: support.apple.com/kb/HT1222