The phrase "beta update" refers to making improvements or fixing issues in a software or app before it is officially released. It is like a test version used to gather feedback and make necessary changes for a better final version. Full definition
Since then, it's released multiple beta updates for both smartphones, the most recent being beta 4. (xda-developers.com)
They can report issues through the app, and can also quit the beta program by sending a waiver request if they no longer wish to receive beta updates. (xda-developers.com)
The fourth beta update came along with an issue that begun as something minor but later on grew its magnitude, hence forcing the company to step in with this fifth update. (nashvillechatterclass.com)