Event posts (text, photos, and videos) are organized under the 'Post' icon based on the date and time they were posted to that event page. The most recently published posts will populate from the top; older posts will appear towards the bottom.
To change the order of posts, you can adjust the publish date/time of the post(s) you want to move.
In the Posts drawer, hover your mouse over the post you would like to move and click the gearwheel that appears 9in the upper right corner. Then click 'Edit.'
At the bottom of the panel is 'Post Date.' To move an event higher up on the event page, set it to be the most recent time. To move an event lower on the event page, set it to a time further in the past.
Click 'Update' at the bottom of the panel. You will see your post in its new position.