To extend the end date of a project, log in as a fundraiser and view your project page as your donors would see it (Projects > click on the name of the project).
Click on Edit details below the Give button. Update the Deadline field and when done, click on Save changes.
Note: When a fundraiser project reaches its end date, all recurring donations are automatically stopped and the donors responsible for them notified. Supporters can, however, still make one-off donations until the project is archived.