Fix error on secured page of website

For one page of my website, I need to change the links in header so that I can avoid following error message in Chrome: ***Your connection to the site is encrypted, but Google Chrome has detected mixed content on the page.** Be careful if you're entering information on this page. Mixed content can provide a loophole for someone to manipulate the page. This content could be third-party images or ads embedded on the page.* Should be a very quick & easy fix, but my web developer is on holiday and want to have done ASAP.