Keeping your Hubs fresh by updating once in a while helps towards increasing your ranking. Google likes fresh content. Ranking tents to diminish over time with stale content that is never updated. So you are on the right track to want to update your Hubs.
I advise not unpublishing first because you may experience a momentary drop in rank and it may take a longer time for Google to recognize that it is back after your republish it. So just update it without unpublishing.
There is an important thing to keep in mind. The way HubPages works, people will see your text in the present form, as you are involved in editing your text capsules. HubPages does an "auto save" every minute or so while you are in edit mode. This auto save actually makes it available to anyone who brings up your Hub in their browser while you are editing.
So my solution is to take the text and copy it into your own text editor (such as MS Word) and work on it off line. Get all the changes done that you want to do. Even spell check and grammar check it, Then, when you are all done, copy and paste the updated block of text back into the text capsule that you are updating. This way, your readers won't suddenly see a weird concoction of unfinished sentences.
Of course, the other solution is to unpublish while you are updating. But then you run into the problem with temporary lost ranking that I previously explained.