Ipod touch will not update

Starting iPod Touch 1st, 2nd, 3rd, 4th, 5th, 6th Generation. If your iPod model mentions, your iPod will not update.

Why??

It is because Apple no longer supports older iPods that mentioned here. Only iPod touch 7th Generation will update in life time.

How to check??

Go here - Identify your iPod model - Apple Support

A 3th generation iPod can't update past 4.2.1

A 4th generation iPod can't update past 6.1.6

A 5th generation iPod can't update past 9.3.5

A 6th generation iPod can't update past 12.5.2