Since you are using only two buttons, you could utilize pin-change interrupts to detect when a button was pressed.
Usually this is the best way to deal with button presses that could occur at any time.
Don't forget to "debounce" your buttons, either in hardware or in your software.
Do this in your ISR:
Additionally you might look into for-loops and the map function:
Might not be very useful in your specific scenario, more for a (battery) graph style LED-arrangement.