Your drive letters will almost certainly vary, but here goes. It's easiest if you put Calbre and its libraries at the top of the drive's directory tree, so you don't have to type such a long string in the command line. In my case, they are both directly on D:/. IMHO, it's also easiest if there are no spaces in the library name, so mine are library and reflibrary. To switch from library to reflibrary, I do the following:
Close Calibre from the system tray
Open the command window - in Windows click Start, Run, and type cmd. Alternatively, push the Windows button and R at the same time to get the Run window.
This will put you at a DOS looking prompt, but don't panic.
In my case, I type the following (push the Enter key at the end of each line):
calibre --with-library d:\reflibrary
To change back to my main library, I do the same thing but substitute library for reflibrary. A key point is that you must be completely exit Calibre to change libraries.