Once you get into the air on the airline and can unbuckle your seat belt and find that you want to move to an empty seat on the plane you can do so as long as you tell the flight attendant first.
Don't just get up and move out of your seat to another empty seat because it could happen to actually be for someone on the plane.
But just ask the flight attendant if you can move to the other seat on the plane and they will usually allow you to do so.
I've done that myself before when I was sitting next to someone who had the flu and was coughing a lot and I noticed an empty seat where I could move over to during the flight to get away from the sick person.
So just ask the flight attendant if you can move to the empty seat and they will almost always allow you to do so.
Also some airlines can move you to another seat from the assigned seat you are already in while on the flight so be prepared for that if you've never flown before.