Open the album in the edit mode by clicking on 'edit' top left. Then click on the gear symbol on top and choose 'sorting'. Change the order of the pictures by date, upload or name in ascending or descending order.

Old editor

Click on 'edit' to open the edit mode, then click on template & sort order on the bottom of the 'edit and enrich'-bar. Choose the way you wish to sort the items in your album:

Chronological (on date and time the photos and videos were made)
Upload (on date and time you uploaded a batch of photos and videos)
Filename (on alphabetical/ numerical order)

Check the preview to see if this is what you like and apply to save your changes.