How Updating a Database Item Actually Works in Notion
Since a database item is itself a page, updating one uses PATCH /v1/pages/{pageId} with a properties object -- the same endpoint used to update any page's properties, not a database-specific update call.
Real API Details
This only changes the item's properties (the structured database fields) -- it has no effect on the page's body content blocks, which have to be modified through the separate blocks endpoints instead.
Practical Considerations
An update call also can't move an item to a different database -- a database item's parent database is fixed at creation and isn't something this endpoint can change.
Use cases
Sync a status change from an external system into the corresponding Notion item property.