Hi td1026 - It's very easy to update or edit in plant files.
If you go to the plant you commented on & scroll down to the bottom of the page, you will see a box that says, "Your Actions". One of the items on the list allows you to edit your comments. You can also edit or "update" at any time . . .for instance if you changed your mind about a plant or maybe it didn't bloom at first, but after a couple of seasons it did.
Just click on the link at the top of any page (under the Dave's Garden heading) that says "My Info". Scroll down the items on the left till you see the one that shows your plant files posts (or bird files, etc.). When you click on that link it will take you to a list of all the plants you've commented on.