Of course if you have forgotten a connection password to Team Viewer you could just ask for the current dynamic password from the user to connect, but what if you wish to change to persistent static passwords but forgotten the "Options" password to access the options screen and change the settings?
Well if this is the case you can do it with access to regedit on the clients computer, whether it be via teamviewer and the dynamic connection password, or physically at the clients premises and the computer.
Open regedit and locate :-
HKEY_LOCAL_MACHINE\SOFTWARE\TeamViewer\Version9
Find the key :- OptionsPasswordAES
Delete this key and restart the TeamViewer service if logged in locally or restart the computer if logged in remotely.
When you retart you will be able to access the options section and modify access passwords and any other options you may require.