Step-by-step guide
- Ben Hollenbeck can get you a certificate from a service like Digicert and Yamini Kannankutti (Unlicensed) can Convert the Certificate from .pfx to .ks format
- The Salesforce Administrator must then go to Setup >>Certificate and Key Management and find the expiring certificate. Refer to tutorial
- When you try to delete the expired certificate you will notice that the "Delete" button is grayed out. This is because it is referenced in the Identity Provider.
- As mentioned the Tutorial go to the identity provider and disable the certificate.
- The Salesforce Administrator can then Install the SSL certificate as described below
https://help.salesforce.com/articleView?id=000339841&type=1&mode=1
Finally Merge a complete certificate chain for custom HTTPS domains https://help.salesforce.com/articleView?id=000333504&type=1&mode=1
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