Depends on what mode of gameplay you choose.
Single Player:
Practice/Hotlap/Hotstint, Superpole doesn't have any AI opponents, so you will not be able to change them.
In a Quick Race, Sprint Race, Endurance Race and Custom Race, you can change the settings when selecting the Game Mode. The settings really are in plain sight. You can change the ability to any value between 80 and 100. Personally I don't recommend going below 90 on any of the settings even if you are a 'beginner'.
Championship:
You are able to fully customize your AI opponents in the starting 'Preliminary Settings' screen before you actually launch your new championship, really you can't miss it.
Career:
I've never played this mode so I can't help you there.
I hope this wasn't a troll questio but really unless you've been playing the wrong game mode, it is very hard to miss the option for tweaking AI.
Kind regard