Can I upgrade & How?

Try Connecting it to iTunes & Check for Update

iPod Touch 6 Gen. doesn't Support iOS 6

Depends on which iPod you have.

Identify your iPod model - Apple Support

Maximum iOS Version for iPod, iPhone & iPad @ Everyi.com

No. Only the 4G iPod has iOS 6.1.6 and a 4G can't go beyond that.

Starting when iOS 7 was released, Apple now allows downloading the last compatible version of some apps (iOS 4.2.1 and later only)

Install the latest compatible version of an app on an earlier version of iOS or OS X

https://support.apple.com/en-us/HT201377

App Store: Downloading Older Versions of Apps

https://sites.google.com/site/appleclubfhs/support/advice-and-articles/app-store -downloading-older-versions-of-apps-ios

You first have to download the non-compatible version on your computer. Then when you try to purchase the version on your iPod you will be offered a compatible version if one exists.