If you maintain a blog, chances are that you have some older posts that are in need of updating--be it for factual corrections, in relation to breaking news or other reasons. While there are no hard or fast rules governing the editing of archived posts, Raj Dash says you should ask yourself a few questions before you take the time to make the changes.
First, what impact will changing the blog content have? If it's a popular (traffic-driving) post, will making changes force the blog platform to create a new URL? If it does, your readers may not be able to find it and you'll kill a valuable piece of linkbait. Dash says that you shouldn't hesitate when making factual changes, however (like misspelling a source's name or quoting a specific amount of funding), but do alert readers to the change with some kind of "updated on [date] to reflect [change]" description.
Lastly, updating posts can generate duplicate content issues, since if your blog platform creates a new URL, you may have two almost identical posts. There's also some evidence that the engines may penalize an older page that suddenly shows signs of activity--particularly link traffic--at least until there's enough evidence to prove that it's not being used for spam.