You cannot change the system Bangla font without jailbreaking (not recommended for security reasons).
For the best compatibility and readability, use these fonts: ios bangla font download upd
Looking to get Bangla (Bengali) fonts on your iPhone or iPad? This quick guide covers safe ways to download and install Bangla fonts on iOS, plus tips for using them in apps, and troubleshooting. You cannot change the system Bangla font without