Event posts (text, photos, and videos) are organized under the Post icon based on the date and time they were published, with the newest posts appearing at the top. 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 in 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.