extremely simple list question

Hi all

This is a extremely simple question about changing a single value in a list in a message object.

How know how to easily fetch any value with zl nth.

But I haven't figured out how to keep a list and change only one desired figure at a time keeping the rest of the list unchanged.

Of course i know it would involve the $1. But having multiple combinations, I haven't figure out this probably more than obvious one.

Thanks to all

phil