To change the machine type of a running Compute Engine instance in GCP, you need to first stop the instance, make the change, and then start the instance again. Here are the steps for each method:

1. Using GCP Console:

a. Go to the GCP Console: https://console.cloud.google.com/

b. Navigate to Compute Engine > VM instances.

c. Click on the name of the instance you want to modify.

d. Click the “Stop” button and wait for the instance to be stopped.

e. Click the “Edit” button at the top of the page.

f. Under “Machine configuration,” select the new machine type from the “Machine type” dropdown.

g. Click the “Save” button at the bottom of the page.

h. Click the “Start” button to restart the instance with the new machine type.

2. Using gcloud CLI:

a. Install the Google Cloud SDK and configure authentication: https://cloud.google.com/sdk/docs/install

b. Stop the instance using the gcloud compute instances stop command:

gcloud compute instances stop INSTANCE_NAME --zone ZONE

Replace INSTANCE_NAME and ZONE with appropriate values.

c. Change the machine type using the gcloud compute instances set-machine-type command:

gcloud compute instances set-machine-type INSTANCE_NAME --zone ZONE --machine-type NEW_MACHINE_TYPE

Replace INSTANCE_NAME, ZONE, and NEW_MACHINE_TYPE with appropriate values.

d. Start the instance using the gcloud compute instances start command:

gcloud compute instances start INSTANCE_NAME --zone ZONE

Replace INSTANCE_NAME and ZONE with appropriate values.

3. Using Terraform:

a. Update the main.tf file to change the machine type of the instance:

resource "google_compute_instance" "example" {
  name         = "example-instance"
  machine_type = "NEW_MACHINE_TYPE" # Change this value
  zone         = "us-central1-a"

  # ... other settings ...
}

Replace NEW_MACHINE_TYPE with the appropriate value.

b. Run the following commands to apply the changes:

terraform init
terraform plan
terraform apply

Note that Terraform will detect that the instance needs to be recreated and will prompt you to confirm the action. The instance will be stopped, destroyed, and recreated with the new machine type. Any data on the instance’s boot disk will be lost unless it is a separate persistent disk set to be retained.

By following these steps, you can change the machine type of a running Compute Engine instance in GCP using the GCP Console, gcloud CLI, or Terraform. Changing the machine type may affect the performance, cost, and other aspects of the instance, so ensure that you choose the appropriate machine type for your use case.