I wish to edit a template that is frequently used. How do I preview it so I don't have to publish in order to view the results?
You should use a sandbox for testing templates:
Templates are a very powerful feature of the MediaWiki software used for Wikipedia, but mistakes can be easily made, even by experienced users. Complex templates should therefore be accompanied by sandboxes and test cases to prevent bugs. Temporary sandboxes X1-X20 or the Template Sandbox may be useful places to try new experiments or for users new to template development.
Have a look at How to create /sandbox and /testcases subpages (or the next subtitle for updating existing
/testcases) - I'm intentionally not copying and quoting the method described to here, since it can cause more un-updated content.
It would be better to stick to the main article on that topic and check the current working method for the time of creating/updating templates.