It's a little bit hidden. If you have an aircraft already defined, which I assume you do, you were asked about ADS-B info when you defined your surveillance equipment. To get back there again to add/change ADS-B info you need to get iFly to the point where it prompts you for that info once more:
Tap/click: Menu | Setup | Select Aircraft | Edit, then tap/click on the line that says Surveillance, select "Nil" (which bounces you back out to the main aircraft definition screen again), again tap/select Surveillance, change Nil to what is correct for your aircraft (for instance "C"), and now you will be presented with a range of ADS-B options you can select from.
If this seems to be an arcane way to get to this ADS-B menu, you'll get no argument from me. This is not the first time that this question has come up. I think everyone would be better served if ADS-B was a visible selection/sub-selection on the aircraft definition screen.
When you have iFly file a flight plan, it will populate the correct ICAO plan field with your ADS-B info.