For any post where you have added multiple attachments, you have the option to determine the order in which they will show once published.
To edit the order of your attachments, select and hold down your mouse on the attachment you wish to move. Then drag and drop the attachments into the order you wish for them to show once published.
You can also expand your options with the attachment by clicking on the three dots in the bottom right. You will see a ‘X Remove’, ‘^Up’, ‘Down’. The up and down option will edit the order of the attachments. The X Remove will delete the attachment from the post entirely.
To edit the order once published, you will first need to edit the post, then scroll down to your attachments where you can follow the instructions above.