Should I change the dirt in flower pot that's been used already?

You can use it for the bottom then use fresh soil to fill the rest.

depends what grew in it and if it died or just outgrew pot. If it died I would say no as it may be you have some sort of soil born disease.

Depends on how old it is, i.e. how long was last plant in it and was it healthy. If only a few years I have reused. Sometimes I mix it with new about half/half to make sure it has good healthy soil. I've done with flowers and vegetables and had good results.

Take out any dead roots/leaf matter and mix with new soil.

It is always best to clean your pots out well and refill with fresh soil so as not to spread any possible disease.

I take out the top half of the pot, removing anything from last year, fill with fresh soil mixing a little and have had good luck. The exception would be if you had anything that died from a disease or for some reason you could not identify, or if the pot is small, like maybe a quart or smaller, then I would remove the dirt and start over. If the plant from last year died of a disease, please wash the pot with soap and water rinsing well and scrubbing off the old dirt.

I dump everything out of the pot into a garbage bag and close it. I give the bag a good massage that loosens the soil from any leftover roots, pick them out and mix it with new Scott potting soil (it contains fertilizers). This year I'm going to add some Soil Moist to hang onto the moisture a bit better. After cleaning the pots, I'll refill and plant my baskets. I change for all new soil every few years. Farmers don't change their soil, they add nutrients, rotate what they plant, so I wonder how necessary it is...except for those who sell potting soil?

Potting soil can be sterilized by placing it on a covered baking sheet in the oven at 2:50 for an hour and then allow to cool to repot