HI9 Plus Firmware

Hi,
my hi9plus tablet shut down automaticaly and when i increase brightness bettery keeps down to 0% and shutdown
is this need firmware upgrade?

if yes then could you please provide me latest firmware

Serial Number : HI9PLUSQ6G18100110
Thanks

To rule out a battery problem, do you do it anyway when the charger is connected?

no ,
how can i buy battery or replace it?