It can happen that your TeamViewer instance is not starting any more after switching over to the new interface or that you really do not like the new looks of the new interface.
- Windowskey + R
- Type RegEdit and press enter
- Copy the following line and paste it into the “address bar” of RegEdit and press enter.
- Double click on the entry named “UIVersion”
- Change the value 4 to 2
- Press OK