The mapped drive should be showing in 'Computer' under windows.
Simply remove as you would a normal mapped drive using the standard windows process.
On reboot of cmx it will error the first time, close the software and re-open it and the error should clear.
There is an alternative way to do this using DOS commands, please follow the steps below:
Bring up a 'CMD' prompt then
TYPE: Net use <driveletter>: /delete
will disconnect the drive
If you do net use /? it will give you a list of options that you can use.