1. Right-click your desktop and choose Radeon Settings.
2. Click on Preferences --> Additional Settings --> Power --> Switchable Graphics Application Settings.
3. Select Fortnite from the list of applications.
Note: If it is not in the list, click the Add Application button and select the *.exe-file of the game from the game's installation directory. This adds the game to the list and you'll be able to select it afterwards.
4. Under Graphics Settings set the High Performance profile to Fortnite
The 'switchable' function has been removed from the graphics drivers for Win10 starting with the new 'Adrenalin 2020'. The option is now located under 'Display' of your computer...."Graphics'. "