Apple released watchOS 5.3.1 on Monday, an update with no patch notes and no security patch notes—which means it keeps Apple Watch up-to-date with iOS 12.4.1, which was also released Monday.
The update notification in the iOS Watch app says that it, “provides important security updates and is recommended for all users.” However, Apple’s Security Update page specifies, “This update has no published CVE entries,” and there is no actual security document for watchOS 5.3.1.
You can update to watchOS 5.3.1 through the Watch app.