Let's change our instance's name to be hello-updated-instance:

resource "aws_instance" "hello-instance" {
  ami = "ami-9bf712f4"
  instance_type = "t2.micro"
  subnet_id = "subnet-5f22f536"
  tags {
    Name = "hello-update-instance"
  }
}

Before we actually run the update, wouldn't it be useful to see what exactly Terraform would do when we run the terraform apply command again? Luckily, there is the terraform plan command that does exactly the same, that is, it shows you what applying would do by checking the template, state file, and actual state of the resource:

$> terraform plan
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but
will not be persisted to local or remote state storage.
aws_instance.hello-instance: Refreshing state... (ID: i-119a10ac)
The Terraform execution plan has been generated and is shown below.
Resources are shown in alphabetical order for quick scanning. Green resources...