In the case of multiple authors, the authors can be ordered using the "Move Author Up" and "Move Author Down" buttons found at the bottom of the individual Author submission form.
Clicking "Move Author Up" will place move the author position up one level switching place with the author above.
Clicking "Move Author Down" will place move the author position up one level switching place with the author above.
This change will automatically be saved to your submission. There is no need to resave the author for it to take effect.
More information: How do I change the order of authors (before publication)?