Step 1 - Select the board you want to edit
First you need to go to the board that you would like to edit by going to Settings > Boards and then pressing on the board that you would like to work on.
Step 2 - Drag and drop your cards in the desired order
All you have to do is to tap on one of the image thumbnails (cards) and keep your finger pressed for a second. Then start to drag the thumbnail into the desired place. Cards are organized from top to bottom and from left to right. So the first 4 items appear in the first row from 1 to 4.