CRT to .CER  

The file extensions .CRT and .CER are interchangeable. If your server requires that you use the .CER file extension, you can change the extension by following the steps below:
Using a Windows based computer

• Double-click on the yourwebsite.crt file to open it into the certificate display.
• Select the Details tab, then select the Copy to file button.
• Hit Next on the Certificate Wizard.
• Select Base-64 encoded X.509 (.CER), then Next.
• Select Browse (to locate a destination) and type in the filename yourwebsite.
• Hit Save. You now have the file yourwebsite.cer