See also the instruction on Youtube here.
Start at the Home screen. Pressing "EXIT" several times should always bring you there. On OpenTX 2.1, you need a long press "EXIT" to return from the telemetry pages.
Go to the Model Selection Screen (push "MENU" once)
go to an empty slot
long press "ENTER"
and select "restore model".
A popup will apear with all avaiable models
Then select a model starting with "Z20" if you are on OpenTX 2.0.
In al other cases, select the model starting with your radio type code, "X09" for the Taranis (X09D, X09D+ or X09E), "X07" if you are using an X7 or X7S (X07 or X07S). For 'Horus' type radios, check Installing a predefined model on a 'Horus' type radio.
Make sure you activate the AppMan model by scrolling down
long press ENTER again
and choose SELECT and press "ENTER"
The star before AppMan indicates it is the active model. Since the AppMan model was preconfigured for using the AppMan, you can now also check if the AppMan is properly installed.
Long Press EXIT until you are at the home screen again.
Now long press "PAGE" to start the AppMan.
In this example, there are no other Apps than the AppMan itself installed. Any newly installed Apps will show up automatically.
Have fun and