Saitoh00-
A few steps are involved but it's pretty straight forward.
Step one... I would update and upgrade raspbian.
sudo apt-get update
sudo apt-get dist-upgrade
Step two...
Compile AM, you can pretty much cut and paste the cmds from the wiki but make sure you read the additional notes.
Follow the wiki
here -> https://github.com/mickelson/attract/wiki/Compiling-on-the-Raspberry-Pi-(Raspbian-Jessie)Don't forget to do sudo ldconfig, inside joke here.
Step three... optional.
Upgrade retropie by running this cmd: sudo ./RetroPie-Setup/retropie_setup.sh
I'm assuming you've already expanded your partition, if not expand it using sudo raspi-config before the next step.
1. Update the retropie setup script first.
Select U.
2. Once the script is done, reboot
3. Once it reboots, run the setup script again
sudo ./RetroPie-Setup/retropie_setup.sh
4. Select 6, update retroPie binaries.