It turns out it’s pretty simple.

Log in to your dashboard (adminstrative system for WordPress). Click on Manage and then “edit” the post you want to unpublish. One of the boxes says “Post Status”, choose the “Draft” radio button, then “Save” as usual.

That’s it. As they say in WordPress-land “were you expecting something else?”.

If you want to change the date of publication then use the “Post Timestamp” – this is useful if like me you are adding quite a few older pages / posts from some other website or blog.