Software

What type of certificate is it then and maybe we can help

You can either purchase a certificate from a Certificate Authority (VeriSign, Thawte...) or create a new self-signed certificate. You cannot modify an existing certificate.

From the Exchange console first create a new certificate using the New-ExchangeCertificate cmdlet. Then you need to use the Get-ExchangeCertificate cmdlet to find the thumbprint for the certificate you just created (be sure to not use the already expired one). Next, use the Enable-ExchangeCertificate cmdlet to make the server use the certificate for the appropriate services (IMAP, SMTP, POP, and IIS if you are using OWA). Lastly, use the Remove-ExchangeCertificate cmdlet to remove the old, expired certificate. You can get specific syntax from Microsoft's Technet site. Use your search engine of choice and the cmdlet name.

I hope that helps. Thanks.