watchOS 4.2.2 can be downloaded through the dedicated Apple Watch app on the iPhone by going to General --> Software Update. To install the new software, the Apple Watch needs to have at least 50 percent battery, it needs to be placed on a charger, and it needs to be in range of the iPhone.
During the beta testing period, no major outward-facing changes were discovered in watchOS 4.2.2. According to Apple's release notes, the update "includes improvements and bug fixes." No other details are available.
For more on the watchOS 4 operating system, make sure to check out our dedicated watchOS 4 roundup.