There are no supported solutions for renaming DB links in 12c without knowing the password.
- If I know the password I can drop/recreate it with a new name.
- If I don't know the password, I cannot use anymore the syntax 'identified by values' because it is an invalid syntax since 11.2.0.4. The expdp/impdp does not offer the possibility to change the dblink name (AFAIK).
The only solution that I have found is not supported, I would prefer an official way to do it.