How to update existing resource for e.g. for resource "aws_redshift_cluster"?

How to update existing resource for e.g. for resource “aws_redshift_cluster” ?

I want to add/create User Database (‘dev’ is default) and add users to this cluster. How I can do it? When I try to do this ‘terraform plan’ shows it is destroying existing one created it. I do not want to destroy existing cluster but want to add these on the top of it. How can we achieve this? Appreciate your cooperation. Thanks, Ravi