I need help in understanding the diff between shutting down a d/b and shutting down an instance.

When we startup an instance using startup nomount, we have an instance in the memory. Only when we perform alter database open will the database actually get mounted in that instance.

Now, how can I dismount/close the current d/b and open another one in the same instance w/o shutting down the instance.

I'm learning the basics of Oracle. So any help will be welcome.