If you still use the default flight planner, then after having pressed the "Find Route" button you're brought to the map, under which there's the dial for the cruising altitude. Change it there.

If you have evolved beyond the default, you must tell me which flight planner are you using.

Ugh! If this doesn't teach me to answer a question in the first hours of the day after an all-nighter, nothing ever will.

Sorry, mistook your question. Yet you did not indicate what kind of approach you use into making AI flight plans.

If you use the manual editing of the flight plans, then in you flightplans text file you'll have a string like this:

AC#1,AZ203,50%,24Hr,IFR,21:00,,

370

,F,0203,LIRF,09:00,,380,F,0204,VHHX

Where the highlighted part states the flight level the AI plane will fly at.

If you use some program that helps you without having to loose your sanity over text files, then... I don't know... never used one... always used the text method myself... that's why I have no sanity anymore.