Hi joanne.goodwin 
How do I ensure that students cannot see the course while I am updating it? I find the publish/unpublish sign confusing.
If your course is published and your course start/end dates are set properly via your course "Settings" page, then your students will be able to access your course. If your course is in an "unpublished" state, then your students will not be able to access the course content. This is one of the main ways that you can control when you want your students to begin accessing your course and submitting assignments to you. You can leave your course as "unpublished" so that you can work on the content...until you are ready to "publish" it. More information can be found in the Guide: How do I publish a course?
Is there a simple way to change dates than going through absolutely every item?
If you are asking about assignment due dates, there is a "Canvancement" (Canvas Enhancement) that was developed by @James which may be of interest to you. It's called Adjust All Assignment Dates on One Page, and it uses a Google spreadsheet. It's a pretty slick tool that James has developed, and he includes a write-up on how it works along with videos to walk you through the process. At this time, James' "Canvacement" is not a part of the course "Canvas" product.
I hope this information will be of help to you, Joanne. Please let Community members know if you have any other questions about this...thanks!
