Is your upstairs MC client computer accessing the downstairs computer as an MC Server, or directly accessing shared folder and files on the downstairs computer?
If an MC client is accessing an MC server, loading the library from the server is a static action. The client doesn't notice any later changes on the server, such as deletion, addition or change to a music file name or folder, or to a playlist, or to tags. There's no way to auto-refresh the library periodically (on my wish list).
To get the upstairs MC client to get the current version of the downstairs MC server's open library, in the MC client click File > Library > Search for Library Servers (even if this option already has a check mark, click it again). If this fails, be sure MC server really is running, the network connection is good, etc, then close and reopen the MC client.
If you not accessing MC server but are trying to directly use files and folders across the network, the likely problem is similar -- the upstairs client is probably using an old version of the library, whether the library is local or remote.