The watchOS 7.3.1 update can be downloaded for free 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.
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 entering Power Reserve mode.
watchOS 7.3, the prior update, brought a Time to Walk feature for Apple Fitness+, expanded ECG functionality to new countries like Japan, and added a new Unity watch face. Apple is also working on watchOS 7.4, a major update that allows the Apple Watch to be used to unlock an iPhone when wearing a mask.