The official Windows blog post advising that Windows 10 Mobile rollout just started from today, to existing Windows Phone 8.1 devices
“The availability of Windows 10 Mobile as an upgrade for existing Windows Phone 8.1 devices varies by device manufacturer, device model, country or region, mobile operator or service provider, hardware limitations and other factors. To check if your device is supported and other important information, visit your device manufacturer’s website.”
Your specific Windows Phone device may or may not be able to download the Windows 10 Mobile update, so download and check the Upgrade Advisor app to see if it is available:
Upgrade is anywhere between 1.2 - 1.3 GB in size (Alert for 8GB Models)
Eligible Models for this upgrade:
• Lumia 1520
• Lumia 930
• Lumia 640
• Lumia 640XL
• Lumia 730
• Lumia 735
• Lumia 830
• Lumia 532
• Lumia 535
• Lumia 540
• Lumia 635 1GB
• Lumia 636 1GB
• Lumia 638 1GB
• Lumia 430
• Lumia 435
• BLU Win HD w510u,
• BLU Win HD LTE x150q,
• MCJ Madosma Q501
What’s new in Windows 10 Mobile?
For further information, please refer to the below link: