We have already covered a rumor that says Windows 10 Mobile update roll-out for older Lumia devices will start on March 17. Tomorrow is that date, now we have got some new information about the update. Evan blass of VentureBeat has also leaked that the update will come tomorrow. Another information from T-Mobile Croatia via NokiaMob blog also reports about the update. As per T-Mobile Croatia the devices which are gonna receive update listed below.

Lumia 435 SS/DS
Lumia 535 SS/DS
Lumia 635 1GB SS
Lumia 640 SS
Lumia 640XL SS
Lumia 735 SS
Lumia 830 SS
Lumia 930 SS
Lumia 1520 SS

To get this update, users will have to open Settings > Phone update and then tap on Check for updates. After that they will see the Windows 10 update. This is just for the first days or weeks. But later users will get a notification about the update even if they did not manually check for updates. You can either use Upgrade Adviser app also. 

Source : 1 , 2