What can cause the fuel mileage to slowly fall on my Prius Hybrid? Like 45.2 to 45.1 and now 45.0. I have 77000 miles should I change plugs or what?

Many things affect fuel economy:
Driving habits,
Driving conditions,
Vehicle load,
Vehicle condition,
Fuel purchased (brand and/or grade), etc. The list can grow long...
Pinning down the exact cause of your variation in fuel econ will require analysis of everything about your drive.

Winter!

Colder temps will cause the ICE (internal combustion engine) to run more because it needs to be at proper running temp to function as it should. Short trips is a prime example, and tire pressure drop.
Your Hybrid is no different than the dozens I service weekly. Leave the tune up till it's due. 120K.

Many things affect fuel economy:
driving habits,
driving conditions,
vehicle load,
vehicle condition,
fuel purchased (brand and/or grade), etc. The list can grow long....
Pinning down the exact cause of your variation in fuel econ will require analysis of everything about your drive.

Thanks for the answer but we purchased this Prius new in 2008 and me and my wife are the only ones that drive it. This just started and I tried a K&N Air Filter but it didn't change.
We did fix it we purchased a new Hybrid, but to everyone that answered THANKS.