Hi,
Excuse the simplicity of this question:
I installed DB2 Express when I originally had just a DB2 client installed.
How can I decide which one to use? Is it just a matter of changing my path?

regards

On what op sys? If UNIX, simply execute . sqllib/db2profile for the instance that you want to use

Assuming that you are on Windows (as your title indicates) and accepted the default values during installation you don't have to do anything, it's transparent. Both client and database can/will use the same information.

Actually, DB2 installations build on lower level installations: an admin client includes the code for a runtime client, a personal database includes the code for an admin client, a workgroup database includes the code for a personal database, etc.. Therefore, your database installation is both database and client.