QNAP TS-119 Upgrade from 3.0.0 Build 0306T to 4+

Just download and install firmware of version 4.3.3 1677 of 2021-06-08
You can download it from
Qnap website https://eu1.qnap.com/Storage/TS-119/TS-119_20210608-
or from the mirror https://st4ck.info/utils/TS-119_20210608-

Login through web interface of your QNAP and upgrade the firmware from settings
Upgrade itself takes few minutes but if the hdd in it is full with hundreds of GB
you need to wait between 30 minutes up to many hours.
So once the QNAP reboot, wait and don't try to reboot/shutdown

Once status light turns fixed green, you can use QNAP again as usual with same network/user parameters
Everything will be preserved including your data in hdd