How do I fix an expired certificate?

You fix an expired certificate by getting a new one issued — there's no way to 'un-expire' the old one. For SSL/TLS: generate a new certificate request, have your certificate authority issue the replacement (with Let's Encrypt/ACME this is one command or automatic), install it, and confirm the warning is gone. For professional or business certifications: contact the issuing body about late renewal — many allow a grace period with a fee, but past it you may need to re-take exams or re-audit from scratch.

Then fix the real problem — the process that let it expire: automate renewals where the system supports it, and for everything else record the expiry date, the renewal lead time and an owner, with reminders early enough to act calmly. An expired certificate is almost always a tracking failure wearing a technical costume.