Yes, you can change the order of your thumbnails by adding numbers in square brackets to the end of the name of your photo in your tour editor.
For example, if you wanted the first thumbnail to be of the Living Room, you would name the photo Living Room [1]. The number and square brackets will not appear in the tour itself - it will only affect the order of the thumbnails. Please see this article for more information on renaming your 360 photos:
How do I rename a room / photo inside my tour?
If you only number some thumbnails, those without a number will be placed at the end.