Although the recommended way to update your firmware is by using companion, there are situations where you don't want to, or simply can't.
But first, make a backup of all your models, or a backup of your EEPROM.
Then, switch on the transmitter while pushing the trim buttons for rudder and ailerons towards each other.
IF YOU DON'T GET THIS SCREEN, YOU NEED TO DO A FIRST TIME UPGRADE. First Time Upgrade
Select "Write Firmware".
Select the appropriate file in the PopUp.
Please don't mix up the PLUS with the normal version. PLUS is clearly indicated on the lower right of your transmitter. If so, select the PLUS version.
Press "ENT" to start the writing of the firmware
Ok, Suppose you have downgraded from 2.1 to 2.0. Then, you need to restore the EEPROM with your 2.0 models in it. Otherwise, you are done!
Select the Restore EEPROM option.
Seelct the proper EEPROM
After selecting, it will ask you to long press "ENT" again to start writing.
P.S. The JustFly install packages contain the firmware, and when copied to the SDCard will allow a 'Taranis' type radio to run, without sound file support. For proper functioning, you need to install the SDCard content as provided by OpenTX.
Have fun and