If you’re in the screen where the buttons and events like pressed, held, etc. are shown, in the corner there are three dots that opens the SmartThings menu and one of the options is Settings.
Unless the driver of your lights supports that (the stock drivers in SmartThings don’t), you’ll have to use a helper driver like this one [ST Edge] Virtual Dimmer Device Driver . It’s going to be a bit cumbersome but SmartThings does not support dimming by steps… Feature Request: Relative Brightness control / Step Dimming for Lights and Groups