By default, iPad’s app dock has four app icons which are Safari, Mail, Photos and iPod. Unlike iPhone which is limited to maximum four icons on the dock, you can place up to six apps on iPad’s app dock.
To change the app icons on the dock, press and hold any app icon until all icons start to wiggle. You can then hold and drag app icon to the app dock, or move a icon out from the app dock. Press the home button to exit the “wiggle” mode.