You cannot change the order of the emails. It is a fixed order of emails and you have contacts that are working through the series and order so you cannot move them around.
You can add new emails in between existing emails but you cannot move existing emails to an earlier position.
Although, you can change the text on emails. So if you wanted to delete text of one email and replace with the text of another email that is before or after it, you can do that.
But this will not change where the contacts are in the order. They will continue through the series and receive whatever the latest changes are for each email when they reach that particular email. So if you manually move an earlier email to a later position in the campaign, contacts that received that email earlier that are still working through the series could receive it again if you move it to a later position.