Page Comparison

Step-by-step guide

...

1. Ben Hollenbeck can get you a certificate from a service like Digicert and Yamini Kannankutti (Unlicensed) can Convert the Certificate to .pfx format
2. The Salesforce Administrator must then go to Setup >>Certificate and Key Management and find the expiring certificate. Refer to tutorial
3. The Salesforce Administrator can then Install the SSL certificate as described below

   https://help.salesforce.com/articleView?id=000339841&type=1&mode=1

4. Finally Merge a complete certificate chain for custom HTTPS domains
   https://help.salesforce.com/articleView?id=000333504&type=1&mode=1

...