Tap on the "burger icon" in the top left corner, then on "Settings" and on "Sort cards". On the following screen you can sort your cards "alphabetically" or "customized". If you chose to sort your cards "customized" every card will be labelled with an icon with "two dashes" on the right side. Tap and hold on this symbol to drag the card to the position you prefer. Tap on the "check mark" in the top right corner of the screen to save your sorting.