How do I switch between aiming with ADS and normal crosshair in Fortnite?

Chapter 6: Season 1 introduced a feature that lets scoped weapons be used like weapons without a scope--this is called canting. Canting allows players to toggle between perspectives so that they either aim down the scoped sight (ADS) in first-person or use a third-person perspective to control the crosshairs.

To toggle your perspective, aim with L2/LT/Right-Click with a scoped weapon, and then click in L3/Shift.

For mobile, a dedicated aiming button that looks like a crosshair with a - on the upper right will appear when you press the main aiming button. Press this to enter toggle to third-person perspective.