How do I modify a mailing-label template?

Christmas is the time for mailing lists! We have a list, originally a WP6 file I think, that needs text updating every year. Some years, however, there has to be a change in the label specification - the Avery template number. I know this can be achieved by creating a new file with the new specification, and then copying all the addresses across, but I can’t find a way to change the specification for an existing file. What if I’m using ‘knock-off’ labels and the sizing needs to be nudged a bit?

P.S. Why did a search for ‘Avery’ come up empty handed? It IS part of the ‘file’ ‘new’ ‘label’ menu.