How to modify UE4 device controls?

I’m developing a game in which the main platform will be Android and iOS. When you start the game you see the following screen:

You can see both default controls for Android devices: the left and the right circles on the screen.

I want to delete the right circle and substitute it with a shoot button. I’ve been searching on Internet, but I can’t find what I need…

Any help will be really appreciated.