Yes. An email campaign can be edited any number of times by clicking on the Edit icon against the campaign name.
If a campaign was created by choosing On a Date condition (i.e Date based campaign), it cannot be changed to On a Condition (Trigger campaign) and vice versa. You can make any other changes that you want.
For a Date based campaign, if the campaign was already launched and all the emails have been delivered, editing the campaign would make no difference, it won't launch the campaign again as the emails have already gone out.Hence, editing a date based campaign only before it gets launched will work. Trigger campaigns can be edited and criterion changed any number of times.