If mi3 comes with dual boot preinstalled, it means mi3 have 2 rom (read only memory/memory to install system ), isn't it? Or another/secondary rom installed on SD card?
Open manager file ( I use es file explore) choose ur .dng file, it will automatically open ur picture on PSE, so u can edit ur picture/s directly from the phone | sorry bad English
Download full rom 4.8.8 in xiaomi website, put on SD card(internal memory) rename it as update.zip
Turn of ur phone, boot on recovery mode chose instal update.zip then reboot