The key's human-readable description can be safely changed using the Key description field. To change the key itself, paste the new private key text into the Private key data field, and the corresponding public key (at seen in .ssh/authorized_keys files) into the Public key field.
Changing the public or private key text for an EC2-registered key is unlikely to work properly, as EC2 stores its own copy of your public key, which cannot be updated by Cloudmin.