You'll need a supported phone (don't go installing this on your main phone unless you fully understand what you're getting):
* Lumia 630
* Lumia 635
* Lumia 636
* Lumia 638
* Lumia 730
* Lumia 830
Don't say we didn't warn you…
Technical Preview is for serious Windows Phone aficionados and tech experts. So if you think S/MIME has something to do with trying to escape an invisible box, you might want to stick with Windows Phone 8.1 for now.
Follow these steps to download Technical Preview:
Sign up for the Windows Insider Programme, if you haven't already.
Find out if Technical Preview is right for you and make sure your phone is on the list of supported phones. Pre-release software doesn't always work as expected and could cause your phone to stop working permanently.
Open this web page from your phone, then click Download the Windows Insider app. You can also search for the Windows Insider app in the Store from your phone.
Once it has finished downloading, open the Windows Insider app on your phone and follow the instructions to install Technical Preview.
Download link: http://go.microsoft.com/fwlink/?LinkId=522524
When you update your phone, it might need to restart more than once to complete your Technical Preview installation. You might need to wait up to 60 minutes for an automatic restart to happen before your phone gets the final Windows 10 update.
Source: Microsoft Web