1. Login to the pfSense firewall
2. Navigate to System -> Certificates
3. Click on Certificates
4. Click the 
"reissue/renew" icon on the certificate that has expired
5. Make sure to check "Use the existing key" and "Use the existing serial number" to keep the OpenVPN server working the same as before.
6. Click on the "Renew/Reissue" button
Full documentation regarding Certificate renewal:
https://docs.netgate.com/pfsense/en/latest/certificates/renew.html