Terminology
.IPSW files are the most robust and reliable method to upgrade/downgrade your iPod. You may restore from an .IPSW file in iTunes using ⇧ Shift+Restore with an iPod plugged into your system, navigating and selecting the .IPSW file and iTunes will deploy your firmware of choice onto your iPod.
The second best alternative to an .IPSW file is to use an iPod updater. Updaters are bundles of iPod firmware(s) accessible through a GUI packaged by Apple to issue updates and behave ultimately the same as .IPSW file. A more comprehensive list of iPod updaters can be found on it's respective article.
Older iPod firmware that is not longer seeded by Apple nor packaged in an updater can be sourced through iPod OS filesystem dumps from existing iPod devices which have yet to update from their respective versions. Such firmware is most often identified by a .bin file extension.
.binimage files prefixed with
must be installed iPodWizard.
.binimage files prefixed with
must be logically dumped onto an iPod using the dd utility.
Methodology
When operating with older iPod touches and devices running iPhoneOS or early versions of iOS (iOS<=5), it is highly advised to be using an older build of iTunes. The recommended range of versions are iTunes 9.0 through 11.1.5, with the most recommended version being 11.1.5. A comprehensive list of downloads are available at the iPhone Wiki.
iPodWizard operates best under Windows XP.[example needed]
Firmware List
Prior to the iPod touch (4th generation) and up until the introduction of A4 series chipsets into the iPod lineup, Apple had yet to implement any restrictions to lock users away from flashing unsigned .IPSWs onto iPods using iTunes. On devices featuring the A4 chip, flashing unsigned .IPSW files had been restricted and would be impossible, meaning you would have to restore your device to the latest signed version provided by Apple and to seek recourse through jailbreaks and experimental workarounds. On devices preceding the A4, the iPod touch (3rd generation) and below, you may interchangeably use unsigned .IPSW files according to the device.
You may also find various sources and types of firmware for your iPod below. Firmware files and filesystem dumps are categorized as follows:
- iPod_
Model Number.Version Number.ipsw or...
- iPod_
Model Number.Version Number.bin
- e.g. iPod_4.1.2.1.ipsw refers to: a firmware file for a 4th generation iPod, containing version number 1.2.1 and it is an .IPSW file, indicating it can be restored in iTunes.
In this table, the .IPSWs you can no longer restore to are prefixed with 
.
Those of which you can restore from are prefixed either with 
, indicating it is unsigned but restorable and those prefixed with 
indicate that particular .IPSW still signed by Apple and still restorable.
Credits
- ↑ 1.0 1.1 Credit to u/UKMATTNZ
- ↑ 2.0 2.1 Credit to Discord user Cosmic#0782
- ↑ 3.0 3.1 Credit to u/Shahid89
- ↑ 4.0 4.1 Credit to Discord user Archimedes#0157
- ↑ 5.0 5.1 Credit to u/NekoMichi