Hi, Anamte3.
Thank you for visiting Apple Support Communities.
I understand that you are attempting to update your iPod touch and need some assistance as the process has changed with iTunes 12. I had the same issue at first and will point you in the right direction. Since your are familiar with the update process, I will include the article that should best assist you. The article below isn't specifically labeled for the update process. However, it does explain how to access the summary screen to update your device.
Open iTunes. Make sure that you have the latest version.
Connect the iOS device to your computer using the included USB cable.
Locate your connected iOS device.
After clicking on your iOS device, tabs appear on the left side of the iTunes window under Settings.
Sync your iPhone, iPad, and iPod with iTunes using USB
-Jason H.