Hi Bella,
Perhaps the easiest way to go about this is a diode on each +V power, they’ll effectively allow either/both of the supplies to power the Arduino. Use a diode that can handle the power consumption of the circuit.
There are lots of ways to go about this, some that would use less energy, I am just aiming for the easiest solution 
Here’s a quick simulation with Proteus (will be less blurry if you click on it to view full size):
