How do i update my apple ipod device to ios 15

No. Unless you have a 7th Generation iPod touch, you cannot update past iOS 12.5.6.

What issue didn't you have before?

Your other iPod likely supported updating to the iOS versions that were available then, nothing strange about that, at some point even that model would have been unable to update any further. Or this current iPod is somehow an older model than the one you had before and cannot update anymore.

6th Generation iPod touches can't update past iOS 12.5.6. It is the last iOS version the support. They lack the hardware for anything newer.

What iPod model do you have exactly? click here ➜ Identify your iPod model - Apple Support

Only the 7th Generation iPod Touch can update to the current iOS 15.7.2

If you have an older iPod running iOS 12, it's probably a 6th Generation iPod Touch, and cannot be updated as it lacks the hardware, RAM and CPU, to run anything newer.

Rivky is correct. The new version of Instagram requires a newer version of iOS to work. However, If you are using the same Apple ID on the new iPod as on the old one, you may be able to get a compatible version of Instagram on the new one, by downloading it from your Purchase history.

It should detect the incompatible iOS version and offer to download a compatible Instagram version for iOS 12, if one is still available.

click here for details ➜ Redownload apps and games from Apple - Apple Support

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rivky33 wrote:

The other one that i have is also a 6th generation and i was able to have instagram

That's because an older version of Instagram worked on iOS 12. But their current version requires iOS 13 or later