Use the browser developer console to identify resources requested with http://, then update them to HTTPS-capable URLs.
Step by Step
- Open the HTTPS page.
- Open browser developer tools.
- Review Console and Network messages for mixed-content warnings.
- Find the source of each http:// resource.
- Change the page/theme/configuration to use https:// or a relative URL when the resource supports HTTPS.
- Retest the page.
If That Doesn't Work
- Do not simply suppress browser security warnings.
- Third-party resources that do not support HTTPS should usually be replaced.