Since the myCloud Desktop Client always synchronises your data with myCloud.ch, thereby always keeping your data up to date, it’s not recommended to disconnect your account and thus log out.
If you wish to do so anyway and would like to link to another myCloud account, you can log out. Simply click on the gear icon in the upper right in the myCloud Desktop Client and select “Settings”. The profile view should then appear, where you can click on the button “Connect another account”. Once you’ve clicked here, a confirmation screen will appear before you are successfully logged out.