I've figured out how to fly in Minecraft Pocket Edition but how do I change the altitude of flight? I'm using the Android version on a Nexus 10.
To start flying using the touchscreen controls, double-tap the center (jump) button. Its icon will change to a wing.
To fly up or down, swipe from the center button to the up or down arrow, and hold.
There are a variety of other interactions among the direction buttons which can be useful shortcuts, but are not necessary like this one; I suggest playing around.
(Source: experimentation. Version 0.6.1 alpha.)