The old flush valve will spin out. I have not tried to see if the new one just spins into the same holder at the bottom of the tank, but I would try that. And if that works let us know.
Otherwise, you will need to shut the water off to the toilet first. Actually you will need to turn off the water anyway.
Disconnect the supply line at the shutoff.
To remove the tank, spin out the flush valve to give you more room for your hand and a screwdriver, and unscrew the holding screws for the tank.
And then lift up and off.