What is a space key ? 

Each Confluence space has a space key, which is a short, unique identifier that forms part of the URL for that space.

The space key can not be changed

How can I find the space key of an existing space ? 

 

With the URL 

The Space key can be found in the URL of the space

In this example the Space key of the space My Business Apps is ISAPPSUP : 

 

In the space details

To edit a space's details:

Go to the space and choose Space tools > Overview from the bottom of the sidebar

 

In the Space Details, you can find the Space Key