Since an airplane has no brakes, how does an airplane slow down?
There are a few ways: change thrust, change air resistance, change elevation.
Have you ridden a bicycle? The flight of an airplane can be approximated by the experience of riding a bicycle.
So, how does someone on a bicycle change their speed?
One way is to simply change how hard you are peddling - peddle faster and go faster, peddle slower and go slower. The same goes for an airplane - they can increase or decrease the thrust and change the speed of the airplane.
Another way someone on a bicycle can change speed is to increase or decrease their wind resistance. If someone huddles over the handlebars and makes themselves small, they'll go faster. If they sit upright so that their whole body is exposed, they'll move slower. Same thing with an airplane - an airplane that faces into its direction of travel (and therefore present less air resistance) will move faster. And when an airplane is tilted away from the direction of travel, like when it pitches upward while landing, will slow it down.
Airplanes also have flaps that can increase or decrease air resistance.
Yet another way to change speeds on a bicycle is to go up or down a hill. An airplane can do the same thing - as it starts to fall back towards Earth, the speed will increase and as it climbs, the speed will decrease.