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.)
To start flying, double tap the jump button when in a creative world. To move up or down, press and hold the jump button, then move your finger up or down.
To start flying, double tap your 'jumping' button. In order to go up, you press and hold the jump key. And, in order to descend doubble tap and you will fall out of the sky.
(Source: Samsung Galaxy tablet minecraft Pocket edition full version)
You can only fly in creative so once in creative double tap the center then you will be in air. If you hold the center and swipe up then hold you should go up. If you want to go down either double click the center or hold the center and swipe down.