You have updated some of your website’s image or CSS files but even after hours or days, the Varnish cache still delivers outdated content.
A common cause for this behaviour is a setting in Drupal’s default
# Cache all files for 2 weeks after access (A). ExpiresDefault A1209600
You should probably lower this TTL (“Time To Live”) setting to a more reasonable value, for example 4 hours (= 14,400 seconds):
# Cache all files for 4 hours after access A). ExpiresDefault A14400