How to upgrade OS ONTAP version from 9.3 to 9.4p1 if any best practice documents step by step then share with us?
You can use Automated NDU for the same. Please refer to Upgrade Express Guide for details.
Please generate an upgrade advisor plan for your cluster. it will give you detailed instructions:
You can also use the upgrade guide in the specific version download page.
Starting in Ontap 9.4. the update should be easier without an external HTTP/TFTP server...