Changing the default generic message

Hi, I know there is a way to do this but I’ve forgotten how… How to I go about changing my Default message to one that I personalize, the one that is used every day. Then, for custom, I would use that for when I’m vacation and leave a message about who to contact, etc. 

I am a Laryngectomy (no Voice Box) so having to switch messages all the time would difficult and I would have my daughter record the Default/everyday message for me. And I when I have a day off, I can do that custom message myself but this way, I’m not always changing the message. 

and where do I find my 4 letter password that I supposedly send up when I bought phone? My  last 4 digits of cell is not doing it… 😞