Whenever I use the Nintendo Switch undocked (handheld mode), it is possible to change a setting with a slider by using the touchscreen. However, when the Switch is docked, it is not possible to use the touchscreen, therefore, I cannot change the slider setting. I've tried changing the setting using the arrow keys on the left joy-con, the left thumbstick, and the right thumbstick, but the only way to change the slider was using the touchscreen. I usually play with the Switch docked, so it would be a waste of time to have to pick up the Switch from the dock and use the touchscreen to change a setting.
I have uploaded a picture in case you don't know what I mean by "setting with a slider." The setting highlighted green as shown in the picture.
Just push "A" on the controller (unless you swapped A/B controls, then push "B"), then you can use the slider.