Troubleshooting
I've Reimported a Certificate but the frontend is still showing the old one
First of all, it can take a few minutes to an hour to propagate. Be patient.
Try incognito mode in your browser in case it's cached it.
Did you definitely Reimport a Certificate in the correct region (US-EAST-1)?
Check the CloudFront Distribution for the website and make sure the SSL Certificate attached there is linking to the one you just re-imported.