Windows 10 Mobile Update is not coming for older phones. However, the hackers are able to build a Custom Windows 10 ROM for Lumia 625. Installing the ROM will bring your device to Windows 10 Mobile Build 10586.494.
You can flash Windows 10 ROM on your Lumia 625 using WP Internals tool. However, flashing a custom ROM could install malware on your phone and the phone could also brick if something goes wrong. If you are still interested in flashing Windows 10 on Lumia 625, you can learn more about it here.
The ROM can be downloaded from here.