I have IPOD model PA623LL at version 3.1.1 (7E18) can this be upgraded to a newer version

That’s a first-generation iPod touch from ~2007 or ~2008.

iOS version 3.1.3 (7E18) is the last version listed for that device.

There is no hardware upgrade, and there’s no software upgrade past 3.1.3.

You'll be looking at a newer iPod touch, or iPhone, or iPad, or Mac, if you want to continue with Apple equipment.

That choice depending on your particular requirements.

No iOS apps presently available will work as far back as 3.1.3.

Current apps with support much earlier than iOS 11 is getting rare.

Other than simple software updates, any 'upgrade' option involves buying a new device. The bonus is that you then get two devices to play with instead of one.

Only to 3.1.3. Use iTunes on a computer.

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