Visit the Index to claim Credits and Exp Bits just for collecting new commanders or leveling up existing ones. Want to know more about your favorite commander? The index is a great place to learn about them.
How to open the index
Tap the drop-down menu on the main hub (highlighted below), select “Index”, and tap on the faction of your chosen commander, select a commander to see more about them. If you have new rewards you will see the target icon on the index button. 
Note: You can also access the Index via the Hangar, by tapping the button in the lower right corner.