This is a really useful function. Sometimes you need to edit your author's comment, date of release, or even the image itself (like if you need to correct a typo). Sure you could delete the page and re-upload it, but then you'd lose any comments you received, and you might also have to spend time moving the re-added page into the right page order. If you edit the page you can just make changes to the one thing you need to.
First, go to that comic's Edit Comic area. You can reach this by clicking "EDIT" next to the comic in your user sidebar:
Then, click on the "pages" button in the Edit Comic area:
You'll see a page which lists all your uploaded comic pages. (If you have a lot of pages, they'll be broken up into paginated lists.) For the page you want to edit, click the "edit" button:
In this example I'm editing the page "2" listed above. Here's the information page for it:
I'm going to change all the information on this page, including uploading a new image (the new image won't be shown, but note the new information next to "choose file":
Make sure you click SAVE PAGE!
And here's what the manage pages area looks like, with the edited information: