How do change a gallon of semi gloss paint to satin.

you cannot ,when the paint is mixed it is a formula based on the finish you have selected

De-glossing paint will reduce it's sheen, but unless you're a pro at this process, it's not a good idea. You can always buy a quart of flat of the same color and mix the two together. That will bring the sheen down. Good luck.

Add 1/2c of baking soda, and a bit of water. or you add 1/2c of plaster of paris and upto 1/2c water for a chalk paint type finish.

Take your gallon of paint back where you got it. Ask them to split it between two buckets, and add what they recommend to make your paint less glossy. You should be able to buy the additional paint by the quart, so the cost should be low. They will mix the paint with a machine and the paint will be smooth and ready to use. Best wishes ☺️

The answer that says you can not do this is INCORRECT. Adding FLOOD Floetrol to the paint will thin the paint, but will also reduce the sheen. The manufacturer's data sheet even confirm this statement:

https://www.flood.com/products/paint-additives/floetrol-latex-based-paint-additive

Cheers,

G