"What I can't understand is how the cabin air pressure changes as all planes are airtight ?"
Cabin pressure changes because an automatic system makes it happen. As an aircraft begins its climb to the cruise altitude the cabin pressure starts to rise so as to maintain a pressure of around 11lbs per square inch, which is approximately what you would feel if the aircraft was unpressurised at 7000 feet.
Your aircraft will cruise at around 30,000 feet.
When the aircraft begins its descent for landing the flight deck crew will set the cabin pressure control system to to the ground altitude at the destination airport, and the automatic system will gradually adjust the cabin pressure throughout the descent.
You are more likely to feel the effect in your ears during the descent. Some people (I'm one of them) never experience any discomfort, but others - like my wife - can get a mild earache for a few minutes. Sucking a sweet can help.
Cabin pressurisation is essential for your comfort during the flight, and is nothing to worry about. Have a good trip.