Microsoft released a new preview of Windows 10 Mobile today. The fourth build for phones includes changes to make one-handed use easier; improvements to Cortana, Photos, and Camera; and a slew of user interface tweaks.

Unlike other releases, build 10136 doesn’t add support for more devices. That said, it does come with a big warning: Because of a migration bug, it is only available for testers with phones that are running Windows Phone 8.1 and opted in to the Windows Insider Fast ring.

As such, if you are running the last preview, build 10080, you will need to go back to Windows Phone 8.1 via the Windows Phone Recovery Tool first. After that, you have to fire up the Windows Insider app and select Insider Fast again.

Oh, and one more thing: “For those of you that do choose to make the jump to this build it is SUPER IMPORTANT TO READ THIS: At the end of the upgrade the Lock screen will appear frozen without the date and time for up to 10 minutes,” Microsoft warned. “Please be patient and let it sit — eventually the date and time will appear and it will allow you to unlock your phone. There is a post-upgrade migration status screen missing, which normally would show you that it was making progress.” If you don’t wait and try to do something with your phone, you could brick it.

With those warnings out of the way, let’s go over the changes. The biggest one is that it’s now easier to use 5-inch and bigger devices using just one hand.

If you press and hold the Start button, your screen will slide down so you can reach items at the top. To slide the screen back up, tap the black space at the top of the screen, press and hold the Start button again, or just wait for it to time out after several seconds.

Next up, Microsoft says Cortana is now “very close to our final design.” Small adjustments have been made to cards in Cortana’s homepage and in the hamburger menu, while the dark theme colors in Cortana’s user interface now match the rest of Windows.

Cortana’s flight and package tracking has been enabled, while alerts for going to work, going home, and the airport have been turned back on. Answers to questions about weather, currency information, stock quotes, and so on now show up inline.

Finally, the Photos app now lets you double-tap to zoom on a photo and tap on a month to get a list of months for quickly jumping to older photos. The Lumia 640, 640XL, 930, Icon, and 1520 phones have gained a new Lumia Camera Beta camera app, which is the Windows 10 Camera app with additional features that light up based on Lumia hardware.

Features aside, Microsoft says this build has “tons of fit and finish changes.” These include changes to fonts, icons, layout, and overall a “more complete” user experience.

Build 10136 also fixes the following issues:

  • The MMS bug in Build 10080, so you should receive MMS messages normally.
  • The bug where touch will stop responding on the Lock screen, preventing you from swiping up to unlock your phone.
  • Visual glitches in Action Center when expanding/collapsing.
  • The bug where the text in the People app was too small.
  • When you toggle the Wi-Fi quick action in Action Center, it now disables/enables Wi-Fi instead of taking you to the Wi-Fi Settings page.
  • Your Start screen background should be scaled correctly now.
  • You can add a detailed status to display on your Lock screen from apps like Outlook Calendar without having the Settings app crash.
  • The issue where a mouse cursor would appear when pressing the back button on your phone.

Windows 10 for phones, or Windows 10 Mobile as it’s now officially called, is behind in terms of features and polish compared to its PC counterpart. Yet Microsoft won’t be launching it until sometime after Windows 10 for PCs, possibly late next quarter, so it has a bit more time. The upgrade issues in particular need to be addressed, as those can’t be as easily fixed via rolling updates that will come thanks to Windows as a service.