In manual mode, the dial on the Nikon D5300 controls shutter speed. To adjust aperture, you must hold down the exposure button while turning the dial. Is it possible to switch this so the default is aperture and the shutter is adjusted by holding down the exposure button? Thanks!
Based on the lack of such an option in the Controls section of the camera's Custom Menu, I would say not. See page 183 in the Nikon D5300 Reference Manual for details.
One possible workaround would be to shoot in Aperture-priority or A exposure mode. In A mode turning the command dial would control the aperture. Pressing the exposure compensation button and turning the command dial would change the exposure compensation setting which would directly alter the shutter speed. In certain specific situations you might need to turn Auto ISO Sensitivity off for exposure compensation to directly affect shutter speed. Of course the beginning point would be dependent upon the meter reading each time you pressed the shutter button halfway.