The labels can be edited from Administration - Labels and Messages. There you can search for Zip and all labels that use it will be returned:http://screencast.com/t/BO97uUHGXTD
. After that simply edit them and change zip to postcode and save. Keep in mind that you need to restart the application before the changes take effect.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items