This is an inbuilt option so you don't need to root your device to change
Font changing is back. But only legacy fonts can be applied.
Note -
1. I am not sure that font changing option is available in stable rom. So if you didn't get the option flash your phone to beta rom and update to its latest version
2 . These fonts are legacy fonts. Which means you can get only those fonts which you downloaded earlier in MIUI 6
Test on
MIUI global beta Rom 6.9.29
Device - Redmi 2 prime
Come back to default font
• Font changing bug in MIUI 8 6.9.29