Microsoft isn't launching Windows Phone 8.1 until sometime this summer but you can get hold of preview if you like. Here's how to download and install the Windows Phone 8.1 developer preview.
Normally registering as a developer to get early access to software is tricky and can cost money. However, there is a fairly easy way of registering with Microsoft for free in order to get your hands on Windows Phone 8.1 today. See also: Nokia Lumia 930 hands-on review: The Windows Phone 8.1 flagship smartphone.
Warning: Before you follow these steps to get Windows Phone 8.1, be aware that it will void your warranty until your provider has officially rolled out the software. Updates will only include the Microsoft update, not additional OEM drivers/firmware. You can't revert to a prior OS version once you have taken an update via the developer program. However, you opt-out of pre-releases to then only receive production Windows Phone updates.
How to download and install Windows Phone 8.1
Head to the Windows Phone App Studio and log in with your Microsoft account. Now start a new project (you don't need to do anything further).
Head to the Store on your Windows Phone. Search for and download 'Preview for Developers'.
Open the app which will appear in your list and follow the instructions, signing in with your Microsoft account and ticking the relevant boxes.
Head to the settings menu of your device, open 'phone update', and check for the latest update. This will now download and install Windows Phone 8.1.
How to enable Cortana on Windows Phone 8.1
It's worth noting that the developer preview does include Microsoft's Siri and Google Now rival called Cortana but is available for US users only. However, there is a way of tricking the device into letting you use Cortana.
In the system settings head to 'Region' and select 'United States' - leave the region format as 'match phone language'. Changing the region will require a reboot.
Now head to the language part of the system settings. Click 'add languages' and select 'English (United States)' to add it to your list. Tap and hold on English (US) and move it up to make it the primary language. You will now find you can switch Cortana on in the application settings.
Once Cortana is enabled, simply tap the search button to bring up the digital personal assistant.