The very moment I started the Tello App it told me about the update and asked me to download.
After that it asked me to connect to the drone, and it installed the firmware.
After that the setting was there.
Hey thanks. Altitude settings wasn't showing in the app earlier but got it now. Actually tello has to be connected to the app and then it shows altitude settings. Otherwise it's not showing there unlike the other settings which are visible any time.
Took tello to 20 but wifi signal drops after. Need repeater for more height.