is it possible to update my old ipod, ver4.2.1?

You can't.

iOS 4.2.1 is the highest version of iOS that a 2nd generation iPod Touch can run.

Nope.

A nearly 12-year old iPod Touch ( 2nd gen ) cannot be upgraded past iOS 4.2.1.

Hardware is simply much too old to handle any newer versions of iOS.

Sorry.