Q: How do I create the CSR and get it installed on Apprenda?
A: To update the platform certificate you will need to renew and download your SSL Certificate from your provider of choice and then upload it to the platform from the SOC (in .pfx format):
GoDaddy has a very good article illustrating the steps necessary to complete the first part (http://support.godaddy.com/help/article/4801/installing-an-ssl-certificate-in-microsoft-iis-7). You will need to follow steps 1 through 7 of the second section.
Once the cert has been imported into IIS, you should be able to export it in the right format through MMC. Make sure to include extended properties and the private key when doing so.
Finally, go to the 'Security' tab in the SOC select your PFX certificate, input your password and hit upload. Once the success message is displayed you should be able to verify that the cert was applied using an incognito window.