The My Personal Credentials feature is a single credential entry which is locally stored on your computer in your Windows profile.
It is typically used to hold the Windows credentials for your running sessions because Remote Desktop Manager cannot access them. If you cannot use integrated security then you must store your credentials in My Personal Credentials.
This allows you to centralize one special credential to replace or emulate the ones for your Windows session. When a password change is needed you simply need to change it once in My Personal Credentials.
If you want to change the credential type, go in %LOCALAPPDATA% (Default) or %APPDATA% (Remote Desktop Services)\Devolutions\RemoteDesktopManager and delete the Credentials.rdt file to reset it.
My Personal Credentials can be selected in your entries under Credentials.