I'm on the .NET/UWP team. The official answer from Microsoft is here:

https://msdn.microsoft.com/en-us/library/windows/apps/dn706236.aspx?f=255&MSPPError=-2147217396

Let me write out the steps from that link above. These steps apply today (May 20th 2015, using VS2015 RC that was released at BUILD.) Deploying to mobile was not possible prior to May14, when the correct build of Win10 Mobile came out.

Your machine should be running Windows 10, build 10074 or higher, from here: https://insider.windows.com/

I installed it "clean" on my SurfacePro1. To do this, first download the ISO of Windows10 from http://windows.microsoft.com/en-us/windows/preview-download, and then use "Rufus" http://rufus.akeo.ie/ to create an installable USB image from the ISO. Within Rufus, for my SurfacePro1, I had to choose "FAT32" and "GPT for UEFI". I don't know what you need on other machines.

Install VS2015 RC from here: https://www.visualstudio.com/en-us/downloads/visual-studio-2015-downloads-vs.aspx

During installation, you get to choose which components to install. You should choose "Windows 10 tools and emulators". Installation takes from one to several hours.

It will install Windows 10 SDK version 10069.

There are some glitches with the installer. If you go back to AddRemovePrograms and modify which components of VS2015 RC are installed, then it ends up installing more than it really should.

When VS runs for the first time, it asks if you want to sign in. I always used to click "no" and it'd then ask me more configuration questions. But if I answer "yes" and I sign in, then it bypasses all those configuration questions, and ends up being quicker. Lesson learned: I now always sign in.

You can run Win10 apps upon emulators. The emulators came with the Win10 SDK, and so are running version 10069 of the Win10.Mobile operating system.

You can run Win10 apps on your local machine. To do this you local machine must be Win10.Desktop version 10074 or higher and must be unlocked.

Instructions for unlocking are at https://msdn.microsoft.com/en-us/library/windows/apps/dn706236.aspx. Here's a "TL;DR" version:

Run gpedit.msc > Local Computer Policy > Computer Configuration > Administrative Templates > Windows Components > App Package Deployment. Then right-click to enable two things,

"Allow all trusted apps to install"

"Allow development of Windows Store apps"

You can run Win10 apps on your phone. To do this your phone must be Win10.Mobile version 10080 or higher (which came out on May 14th). Your phone must be unlocked.

Instructions for unlocking are at https://msdn.microsoft.com/en-us/library/windows/apps/dn706236.aspx. Here's a "TL;DR" version:

Plug your phone in by USB. Run "Windows Phone Developer Registration" tool from your desktop.

The key fact about Win10 is that you can write software against one particular SDK (e.g. the 10069 SDK that came out at //BUILD), and still run it on a more recent version of Win10 (e.g. 10074 that's current for Win10.Desktop program, or 10080 that's current for Win10.Mobile insider program).

In the .appxmanifest file, MinVersion="10.0.10069.0" says that your app will refuse to deploy to any machine which is running a lower version of Win10.

In the .appxmanifest file, MaxVersionTested="10.0.10074.0" says that if your app is deployed to a machine which is running a higher version of Win10, then it might have to do "quirking".

Both of these things are controlled by the Project>Properties dialog, and are actually stored in the .csproj/.vbproj.

The answers by @user4855870, @Rexobias and @Sevenate were hacks. They were sticking in an artificially low version for MinVersion to work around the fact they hadn't yet updated their devices. That's a fair approach (and I did it while I was waiting for Win10.Mobile 10080 build to be released). But now that it has been released, there's no need for these hacks, and you should just update your device.