For those who use the Insider program for Windows 10 Mobile you will want to grab version 184.108.40.206 (up from the previous 220.127.116.11) of the app now available in the Store.
Interestingly, this is a mandatory update. If you launch the older version that is probably now on your phone you will see a message stating that an update is required.
Although no changelog is listed, the update is likely related to the ongoing changes in Windows 10 Redstone. Currently, there are no UX or UI changes in Redstone as Microsoft is doing a lot of plumbing in the OS for new features later on. As such, we do not expect to see a lot of outwardly facing changes for a few weeks.
The Redstone branch (i.e. rs1_) of Windows 10 is only available so far for PC users. This app update, however, is likely prepping phones to go to that branch instead of cumulative OS updates that we have seen lately.
Sources close to Windows Central expect Windows 10 build 10586.63 to be released tomorrow for all non-Insider PC users along with non-Insider updates for mobile users with the Lumia 950, Lumia 950 XL and Lumia 550.
For more information about enrolling and using the Windows Insider program head to https://insider.windows.com/en-us/ or read our previous coverage here including how to install and use the Insider app on your Windows Phone.