First you need to interop unlock your Lumia (running WP8.1 or Windows10 Mobile).
if you don't how to unlock then go here.
1. Now download FontChange.rar from here and extract all files.
2. Install FontChange_Release_ARM_v0.2.xap (DHT FontChange) with App Deployment SDK (8 / 8.1).
3. And copy the font.ttf to SD Card.
4. Now open DHT FontChange app and select font => choose the font and press Ok.
5. Now reboot (Switch OFF/ON) your Lumia.
Thank You
