If you have created a custom certificate key pair and a new passphrase, open the Import Custom Certificate Key tab provided from Configure > SSL > Custom Certificate Key.
Steps
Use Choose File to browse and locate the key to be imported.
The key must be in PKCS#8 format, be base-64 encoded, and 2048 or 4096 bits in length.
Enter a Passphrase and then Confirm Passphrase.
The passphrase and confirm passphrase entries must match and be the passphrase used when the key was created.