Can I, and if so, how do I change the title of a post, after I have published it?
The answer to the first part of that is YES. You can change the post title, after you have published the post - BUT, the URL will NOT change.
Let me explain how. If you publish a post and then decide to change the post title, simply click on Edit Posts, from your Dashboard, select the post you wish to change the title of, and change it! Then, re-publish the post. No worries. The URL, which has already been allocated to that post, will NOT change. I really don't see any need for it to change, anyway, unless there is a glaring error. However, if you do want the URL to change, you have to make a New Post, AND, use a different post title, otherwise, the URL will reflect it. Blogger won't allow duplicate URLs - See: here
eg: - If you do want to change the post title and the URL, start a 'New Post', add the content of the 'old post' (copy and paste), then re-publish with the New Post Title. That will give you the old post content, a New Post Title and a new Post URL. Simple. Then, and don't forget this - delete the 'old post'!