When you are editing the Summary field, you need to use the arrow keys to navigate along that field. It doesn't appear in a popup anywhere so you can see and edit the entire thing at once.
Alternatively, you can write out the revised summary in your word processor, delete any text in the Summary field, and paste in the revised text. If you're making many changes, that may actually be easier than arrow-key navigating.
Just bear in mind that the Summary field only allows for 240 characters, including spaces. If you've reached the limit, it simply won't allow you to add another character, which might appear as if the field is uneditable.