As for your question – go please, to Gallery plugin and check the insert “Galleries”. Click on the button “Image List” near the gallery you need and by clicking on the left button of the mouse change the order like you want.
The other way is to sort them by some criteria, e.g. by the date of posting. For this, you should click the button Sort by and choose the criteria. (You have variants: add date, create date, position, size, name, tags). Look on the screen where to find this button, please – http://prntscr.com/b2pif9
I hope, we helped you. If no, please describe more detailed your problem. Thanks in advance.