At the moment you can only play as Ciri in certain parts of the game, but with the help of a mod that would enable the debug console, you can enter a command where it would switch the main character Geralt to Ciri with all of her abilities (depending on where you are in the game).
Firstly download the mod HERE then install it into your game. You do that by opening the file that you extracted from zip file that you’ve downloaded, copy both files “plugins” and “dsound.dlll”, go to your Witcher 3 game folder then open “bin” then “x64” and after that paste those files that you copied earlier inside the “x64” folder. Note: You can uninstall the mod by remove the same files you pasted.
After installing the mod, open and run the game, and once you’re playing open the debug console by pressing the ( ` ) key or ( F2 ) key then type type in exactly this without the quotations: ” Ciri replaceplayer ” then hit enter. After that you’ll be able to play as Ciri in the open world of The Witcher 3: Wild Hunt.
To reverse the process so you’d be able to play as Geralt again, type: ” Geralt replaceplayer ” and you’ll be playing as the main character again.
Also keep in mind that bugs will appear because of the change in the player character, so it’s best to manually save before installing and/or when switching the main character.