It is possible to change semigloss paint into a satin, but there are reasons not to. Semigloss is a good finish to protect surfaces. It also is cleaned easily. Satin finish is between flat and semigloss. It has a slight sheen. The sheen is more visible from an angle. It is elegant, and works well in a decor where you want a soft sheen. The problem with changing a semigloss to satin is you can't be completely sure how flat or shiny your satin finish will turn out. In addition, you'll need a flat paint of the same color to mix it. The result means that, unless you save some of the mixture, touch-ups later won't match. If you decide to create your own satin paint, put a little aside for later.
Pour the semigloss paint into a bucket. Use half of what you will need. Interior paint covers 400 square feet (typically) for each gallon. For a 200-square-foot wall, therefore, start with 1 quart of the semigloss.
Pour in a matching amount of flat paint.
Mix well. A drill and attachment work best. Submerge the drill mix bit, turn on the drill and mix fully.
Paint a test swatch. Dab a bit on the wall. Wait for it to dry. Examine the finish. Look at it from the side and front. Since it's your custom finish, you can customize the sheen. For a bit more, add some semigloss; for less sheen, add more flat paint.
Re-test to ensure the finish is satisfactory.