I have this problem both VFR and IFR.

I try to request a change of altitude but the say it function allows me only to request the FL originally filed.

~ <CallSign> request new altitude of Flight Level <????>

I tried manually change the text but it's impossible. Say it function continues saying always the same flight level assigned to the original flight plan. Where can I input the new flight level / altitude I want to fly?

Sounds like you're on an old version of P2A that had this problem.

Update to version 2.6.2.3R2 and it should let you request different altitudes in flight.

Dave