Prerequisite

• ubireader
• mkfs.ubifs and ubinize (search how to install them)

Step 1.1: Unpack system_unlocked.img

ubireader_extract_images system_unlocked.img -o system_u
cd ./system_u/system_unlocked.img/ 

Now you will get a file with a .ubifs extension.
In my case, it's "img-35657280_vol-rootfs.ubifs"

Step 1.2: Unpack ubifs

ubireader_extract_files -k -o ./rootfs img-35657280_vol-rootfs.ubifs 

You will now have a folder named "rootfs" containing all system files.

Step 1.3: <Modify Firmware>

Step 1.4: Repack .ubifs

mkfs.ubifs -m 2048 -e 126976 -c 1073 -x lzo -f 8 -k r5 -p 1 -l 5 -F -r ./rootfs/ rootfs.ubivol

Set up the config.ini file:

touch config.ini
echo "[rootfs-volume]
mode=ubi
image=rootfs.ubivol
vol_id=0
vol_size=27807744
vol_type=dynamic
vol_name=rootfs" > config.ini

Change the "vol_size" value when the firmware is modified. Keep it as is unless you encounter size-related errors.

If you do encounter size-related errors, the error will have the 'vol_size' that needs to be set.

Step 1.5: Repack img

ubinize -p 131072 -m 2048 -o system_m.img ./config.ini

You will now have the modified firmware (system_m.img) ready to be flashed using fastboot.

Step 1.6: Flash using fastboot

How to Boot in Fastboot:

• Remove battery from JioFi 3
• Locate reset pin hole button
• Lay down JioFi 3
• Use Paper Clip / SIM Ejector Tool to press button inside (using sharp object is not recommended)
• Keep pressing button down and insert USB cable
• Release button as soon as all the LEDs in Display turns RED. This means device is in boot loader mode

fastboot erase system && fastboot flash system system_m.img && fastboot reboot