You said,
"The environment variable 'path' is set to 'bin' of 12c"
It should be a path that looks something like
C:\app\oracle_account\product\12.1.0\somefolder\bin in windows
or
\u01\app\oracle\12.1.0\somefolder\bin in Unix/Linux
So, your Oracle 12c home should be
C:\app\oracle_account\product\12.1.0\somefolder or
\u01\app\oracle\12.1.0\somefolder\
Now, in windows, check the registry entry ( regedit )
HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\KEY_HOME_NAME
It should be set to the new oracle home discussed above otherwise change it or go to environment variables and set the ORACLE_HOME value.
Now, your PATH can be set using set PATH=%ORACLE_HOME%\bin;%PATH% to point to the correct bin directory.
For Unix, Linux you could export them correctly in the environment which you use to connect to the database.
export ORACLE_HOME='\u01\app\oracle\12.1.0\somefolder'