Of course, when we edit an item of content, the item already exists. Click the edit tab for an item and you will see the data entry panel for the item, along with the existing values of the item's data.
For an example of something really simple, where editing looks the same as adding, we can review how to edit a folder.
The edit panel for a folder simply shows the title and description input areas. Often a description is not provided for a folder, so the only thing to change is the title. If you do wish to give a description, which is a good idea for distinguishing folders in a list, the description can be text only -- there is no opportunity for setting styling of text, such as bold, italics, or other formatting. This keeps the descriptions of Plone content items as simple as possible.
Here is the edit panel for a folder, in this case, one called "Butterflies":
That's it. Change what you want and save, and the content item will be updated in Plone's storage system. You can repeatedly edit content items, just as you can repeatedly edit files on your local computer. By now you have appreciated that Plone stores discrete content items as separate entities, akin to "files" on a local computer, but you really don't have to think about it that way. Plone is a content management system, where the content comes in the form of numerous discrete content items that may be individually edited. Edit away at your heart's content.
For an example of editing a content item that is a bit different than adding in the first place, we can examine editing an image. Editing an Image can be done by navigating to an individual image and clicking the edit tab. Clicking the edit tab for the image, you will see the following edit panel:
Here, an image called Butterfly-small.jpg is being edited. You can change the title and description, as usual, in which case you would usually keep the setting to "Keep existing image." You can also change the image itself by checking the "Replace with new image" choice. Or, clicking the "Delete current image" choice will simply delete the image entirely. The "Related Item(s)" setting at the bottom is the same one always present for Plone content.
So, editing an image is a bit different than adding one in the first place, but not by much.
Editing panels for other content items are straightforward.