In the last couple of test builds for the Windows 10 Anniversary Update, aka Redstone 1, Microsoft has included an option in the Settings app to manage your enrollment in the Windows Insider Program.
In the past on mobile devices you had to download and install the Windows Insider app and then opt into receiving Insider builds in your choice of Fast, Slow or Preview Rings.
Now, in all future Windows 10 Anniversary Update test builds, the Windows Insider app has been deprecated and will no longer be used to manage your Windows Insider Program enrollment. Those options have been moved into Settings>Update & security>Windows Insider Program.
On other builds of Windows 10 for mobile devices the Windows Insider app will still provide you the ability to join different testing rings until you make the move to the Windows 10 Anniversary Update after it is released in July.
For those of you who might be on Windows Phone 8.1 the Windows Insider app will also assist you in getting preview builds on your eligible devices.