When you're finished with a tool, tap the check mark in the lower right corner to accept the changes you've made, or the "X" icon in the lower left to discard the changes you've made. This will bring you back to the main interface. See the screen shot below.
But my screen doesn't look like that.. my tool are on top of the screen and I don't see any "x" or check mark. And when I close Photoshop and re open it the same tool is active!