Can I edit the content of an existing campaign that has already sent emails to some of the contacts?

You can pause the campaign and change the copy. The system will just pick up where it left off with that contact. So if they received email 3 they will get email 4 however many days later. Then the system will use the same sequence for the steps, e.g. if email 4 is a 'reply in same thread' email, then it will use the copy from the new email 3 to create the thread.

Just remember to start the campaign again after making your edits.