How to clear a cache of Web site in server

I developed a website. It has been hosted in google cloud platform.I was required to change the banner of picture of the home page of website. I changed in server but It showed previous banner picture so I cleared the cache by following command

sudo touch /var/cache/mod_pagespeed/cache.flush

subsequently The picture was changed. But After few hours The previous(old) picture again replaced the new picture. What can I do to solve this problem and what is the reason?