We have existing web service client on salesforce to call out third party web service with CA signed saleforce certificate. When this certificate is expired, how to renew it from saleforce "Certificate and Key Management" admin console?
You need dowload the csr from Saleforce (Your name => configuration => secure settings => certificates => create new certificate CA) and send the csr to your CA to sign it. After that you can upload the new certificate.
Remember that the new certificate need to signed using SHA-256, And you need configurate your service to SHA-256 and TLS