Which partition does /boot/exlinux directory belong to?

I modified /boot/exlinux file, then the machine cannot start up. Could you tell me which partition can I reflash to recover this file? If it is the APP partition, Is there a way to modify exlinux file without reflash the APP partition?

By default “/boot/extlinux/extlinux.conf” runs on the root filesystem (no separate “/boot” partition). If you have an SD card model, then this means on the SD card. If on the eMMC model, then mmcblk0p1 (the root filesystem).