It is possible.
Here is the kubernetes secret documentation.
Put your signed cert and private key into the same directory where you self signed cert and key is stored.
If you created your secret using kubectl create secret... command, you can update the secret with kubectl edit secret... command.
If you created your secret using kustomize/delpoy files, update your kustomize/deploy files to point to the new cert and key, and then re-apply your kustomize/deploy file with the kubectl command.