How do i get the best mileage for my 2013 honda fit?

I see most consumer report review on it and they get around 30-31 mpg and i get 24.4? Would it be just needing to replace some car parts on the interior such as the air filter? I have just gotten my tire replaced and upsize so i hope it will make a little difference

That depends tremendously on where and how you drive it. Stop-start traffic is going to use a lot more fuel than a steady long trundle on the motorway, for instance. There are various things you can do to increase your mileage, including not carrying unnecessary weight, ensuring the tires are up to pressure, keeping the windows closed at speed and above all not revving the nuts off it. Whether it would benefit from new plugs or air filter we can't tell, but you could certainly try them if you wish.

A heavy right foot increases fuel consumption. Keep tyres at the correct pressure. Use the correct grade of oil. Drive with windows closed. Change air filter in plenty of time. Anticipate road conditions and accelerate gently into a higher gear whenever possible. Hard acceleration and braking uses lots of fuel.

Change your driving habits.

If you are driving in the city and doing short trips, there's no way you will get to the advertised mileage. Drive on the freeway for 2 hours straight and check your consumption. If you get a lot more than the advertised mileage, there's NOTHING WRONG with your car. If the fuel consumption did not improve, it is time to see a mechanic with a scanner.

How YOU drive will have more impact than EVERYTHING you can replace combined. The mix of highway driving to stop and go city driving is also a significant factor.

Only drive down hill.

IF you have upsized the tire diameter, for each revolution of the axles you will go farther but the car odometer will measure a distance calibrated for the original equipment tires unless you have recalibrated the speedo to match the new tire size. Hence your MPG will be lower.
Other hints to adding MPG can be found if you google "hypermiling" and read some of the tips and tricks.

Larger tires may hurt your mileage. Slightly taller is fine but wider will hurt. Keep them inflated properly.

FIRST UNDERSTAND HOW THEY CHEAT. ALL AUTO MANUFACTURERS DO IT. You say they say 30-31 per gallon. They do their testing on a dynometer on 1 wheel. Not taking into account the rolling resistance of 4 wheels AND wind resistance against the vehicle and brake drag and weight of occupants. You will never get their numbers UNLESS YOU DRIVE DOWNHILL ALL THE WAY @ 55mph with a 55mph tailwind then you MIGHT BE BETTER OR THE SAME AS THEM. Without using your brakes. Chances of that are 0.
Tire upsizing creates more rolling resistance and makes your speedometer and odometer inaccurate.
I had better MPG on a car that had 13 inch wheels than I did on a car that had 15" wheels. Tire quality matters. Meaning brand name tires. Like Michelin or Perelli (not discount tires buy 1 and get 3 for free deals) It matters as much as 7MPG ( though the tire deal savings may just pay for all the extra fuel you will burn)

Have had years and cars to experiment with.

Stop and go driving is the worst for mpg. Change how you drive, perhaps even the roads you go on. Taller wheel aren't going to help with this, since the car has to do the same amount of work going from A to B.