When I try to update to a new iOS 13 version in my iPod touch 128 Mb do not present the update only iOS 12.4.5

If you have a Gen. 6 Touch, then tap on Settings > General > Software Update in order to upgrade iOS. You can also connect the Touch to a USB port on your computer. An entry should appear in the Finder under the Locations heading. If you select the Touch, then the Finder will open it in a window so you can upgrade it from there as well. When you upgrade be sure to connect power to the device if you upgrade by Wi-Fi.

Your iPod Touch is too old to update iOS. It lacks the hardware needed to support iOS 13 and beyond. You need an iPod Touch Gen. 6 or later to run iOS 13.