Hi Trinetra,
It is not the SAP User with which End User Logs on to NWBC page using End User Log On. As Plaban has mentioned, It is the service user, maintained in GRAC_UIBB_END_USER_LOGIN(SICF). To see the user go to SPRO->GRC->AC->User Provisioning -> Activate End User Logon and double click on the service GRAC_UIBB_END_USER_LOGIN(SICF) and go to Logon tab. So the changing of the user's password is not possible through End User Logon.
And the User ID and Password entered by the End User in the End user logon page is the User's LDAP ID and password.
Regards,
Fazil