Apple released watchOS 7.6 on Monday. It accompanies the release of iOS 14.7.
watchOS 7.6 Now Available
The main thing watchOS 7.6 introduces is the ECG on the Apple Watch Series 4 and irregular heart rhythm notifications to 30 more regions. A website lists if which regions these features are now available in.
To update your Apple Watch, open the watch app on the iPhone it is synced with, then tap Settings > Software Update > Download and Install.