Lumia 640XL device users: Unfortunately the information here won't apply to you yet. The resolution scaling bug fix has not been added to a deployed Windows 10 Insider build. Your device is not yet on the approved list unfortunately.
All other 640 users, please continue reading below:
As you know, the Windows Phone OS has been launched on a large number of devices around the globe, and this includes a multitude of countries and mobile operators. Depending on when your device launched, your device could have been on of a variety of specific build numbers.
If your 640 handset is on Windows Phone 8.1 and has an OS version greater than 8.10.15116.125, you will not be able to upgrade to Windows 10 Technical Preview currently. To check the current OS version of your device: Settings > About > (More Info)
I know this news is disappointing. This is a temporary, not permanent, situation. Without going into great technical detail, I will summarize and say that if you are on a build greater than the one noted above and update to Windows 10 Technical Preview,
your device would not be able to recover back to Windows Phone 8.1 if you were to encounter any device troubles or decide the Preview wasn't to your liking.
This is not a situation we want to put users in. Having the ability to recover or roll-back should you choose is important. We are actively reviewing possible alternatives to see how we can best resolve this. In the short term, please know we are working hard to provide a great experience and ask for your patience while we continue working on this.
Thank you for reading. There's more to come, so stay tuned