From the terminal:
$ sudo nano /etc/rc.local
Note the commands to exit and such at the bottom of the terminal window. control + q will ask if you want to save the buffer to file.
You need to comment out with a hash, remove or modify the part where you I assume would see a bit about emulation station. I assume it would be in this file, but there is technically more than one way of starting an application on boot. If you don’t see it, also check /home/pi/.bashrc.
You could probably just add the word attract. Double ampersands are used to execute more than one thing, so take note of that. If it does not work, you might have to use an absolute path. But I don’t see why it shouldn’t. Sorry I can not give you a 100% answer because I do not have your setup.
Reboot your pi:
$ sudo reboot