open save file in text editor (I use Notepad) and search for "human=yes" - this will be start of data of your country in save file

One line up you will see three letter/diggits code and sign of equality with parenthesis - for example: "CAS={", "POR={", "ARA={" or "D00={" - it's start of record of country [those three letters it's country name in save], so when you scroll down save file and you will see another line like this it mean that this is start of record for another country.

scroll down - there will be line "history={" with records of your monarchs, generals, events etc. - each starts from date - like "1357.1.22={" - you search for records with line monarch={. Go to the last monarch record for your country. Each ruler record have date first, next line "monarch={", next lines with his ID, name, from witch country he is, stats, traits and next line with dynasty name - so you can change it

Look at monarch's ID and search if there are other instances of it in save file.