The watchOS 7.3.1 Update can be downloaded for free via the dedicated Apple Watch app on the iPhone by going to General> Software Update. To install the new software, the Apple Watch must have a minimum battery of 50 percent, placed on a charger, and be within range of the iPhone.
According to Apple’s release notes, the watchOS 7.3.1 update introduces a fix for an issue that prevented some Apple Watch Series 5 and SE models from charging after going into Power Reserve mode.
watchOS 7.3, the previous update, brought a Time to Walk feature for Apple Fitness +, expanded ECG functionality to new countries such as Japan, and added a new Unity watch face. Apple is too working on watchOS 7.4, a major update that allows the Apple Watch to be used to unlock an iPhone when wearing a mask.