Thanks for your question, Mintu •
It appears that modifications to "Network Connection" and "Virtual Network" settings for an Azure Database for PostgreSQL Flexible Server (as of now) cannot be update using Azure CLI or other command-line interfaces.
These changes can exclusively be performed through the graphical user interface (GUI) of the Azure Portal.
By the way it can be changed through ARM Rest APIs:
Reference https://learn.microsoft.com/en-us/rest/api/postgresql/flexibleserver/servers/update
Please let us know if this help or if you have any additional questions.
Thank you!