If you have an existing paid Megascans subscription on quixel.com and would like to link your Epic account, just sign in with your Epic Games account. This will link your Quixel Megascans account with your Epic Games account.
Click 'Sign in With Epic Games' and enter your Epic Games credentials to link it to your Quixel Megascans account. You can use your Epic Games account to log in to your Quixel account.
If you have an Unreal Engine license, you'll see an option to switch to the Unreal Unlimited plan. You can also manually subscribe to Unreal Unlimited for free by heading over to quixel.com/pricing and clicking on the Begin for Free button.