How do you make a semi gloss paint shinier?

Add Flat Paint to the Semi Gloss (SG). As a general rule, using commercial "production" grade paint.

If you want "Satin" and have SG. you could add 1/3 of a Gal flat, tinted to the same color.

If you want SG and have full Gloss Paint, Add 1/3 of a Gal Flat, tinted the same color.

The reverse is N O T true.

If you want full gloss and have Flat, it won't work in my experience.

Experiment on your own. Just remember its easier to tone down gloss than to add it.

This is assuming the paint is still liquid.

If it is already on the walls, and you want to avoid repainting. A "Matte" finish Clear Coat could be sprayed over the whole wall, But to do it "right" you would want to repaint