You renew a wildcard SSL in the same way and a normal SSL. From the server where you first generates your CSR you need to generate a renewal CSR. In most cases there is an option to do this in your control panel or server. If you don't have this option then simply generate a new CSR using the exact same details as your original CSR.
Once you have received the new SSL, install in on your server. If you have installed your wildcard SSL on multiple servers you will now need to re-export the new SSL and private key and install this on all other servers where it is installed.
