Create VM Instance

• Name (fixed)

• Region, Zone (fixed)

• Machine Type

• Price

• Boot disk / OS image (fixed)

There are only very few options that are "fixed" once you create the instance. Most of them can be changed later. Some can be changed while the instance is running, others require it to be shut down and restarted.

Look at the prices. They charge by the second, so you can turn on and off instances without using a lot of the credit you have.