Use your normal method to renew your certificate, either internally if you have an internal CA, or externally from a certificate vendor like Verisign, Thawte, GeoTrust etc.
Once you have your renewed certificate, review the documentation in the Configuring SAS Web Server Manually for HTTPS section of the SAS 9.4 Intelligence Platform: Middle-Tier Administration Guide. You will need to replace the contents of the file referenced in SSLCertificateFile and restart the SAS Web Server. If you change your CA and/or intermediate CA then you may also need to need to add their certificates to the Trusted CA Bundle. If you have SAS 9.4 M3 then you can use the SAS Deployment Manager to do this - see the Adding CA Root and Intermediate Certificates section in the SAS 9.4 Intelligence Platform: Installation and Configuration Guide.
This is assuming you terminate SSL at the SAS Web Server. If you also use SSL for the SAS Web Application Server and SAS Environment Manager too then you will also need to look at those. Check the relevant documentation for those.