Neowin has a post up to say they reached out to Microsoft and got a response listing the following devices as the very first that will get Windows 10 Mobile software updates. Take a look to see if yours is listed:
Lumia 640 XL
Note that the above listed phones are not the only ones that will get Windows 10. They are just the first in line.
Read the complete details: Microsoft confirms that these ten Lumias will be first to get Windows 10 Mobile.