The easiest way to install it is to just exit the EAC launcher and run the game again from the Epic Games Launcher and click yes when the EAC install prompt comes up from Windows.
If the prompt doesn't come up for some reason you will have to install it manually following these steps:
- Navigate to the Easy Anti Cheat folder in your Fortnite install directory
[Default is C:\Program Files\EpicGames\Fortnite\FortniteGame\Binaries\Win64\EasyAntiCheat Folder]
- Locate EasyAntiCheat_Setup.exe
- Double click the file
- Click the install Easy Anti Cheat button
- Start the launcher and Fortnite