When you make changes to your event that has already shared on various social media and during the course of the ongoing event, you make changes to the cover image of the event. At times, the change is not reflected as the thumbnail image is cached.
In order to clear caching from Facebook and LinkedIn,
Step 1. Please use the below mentioned URL to debug your event
Step 2: Enter your URL and click on Inspect, You will see the updated preview image.