Why does it say the security certificate isn't valid and how to fix it?

A security certificate may be marked as invalid due to reasons such as expiration, mismatched domain names, or being issued by an untrusted certificate authority. To fix this issue, you can try reloading the page, clearing your browser's cache, or checking your device's date and time settings to ensure they are correct. If the problem persists, consider updating the certificate for the website or contacting the site administrator for resolution. Alternatively, you can proceed to the site at your own risk, but this is not recommended for sensitive transactions.