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.
If you're asking for technical help, please be sure to include all your system info, including operating system, model number, and any other specifics related to the problem. Also please exercise your best judgment when posting in the forums--revealing personal information such as your e-mail address, telephone number, and address is not recommended.
I am running exchange 2007 local certificate expired how can I fix this?