You can't individually change the titles, only the word "King" into Emperor. For example, if you're wanting to make the title "King of Germany" read as "Emperor of Germany", you can do this, but every King title in the game will also be referred to as Emperor.
If you don't mind this, go to your config folder, then "text.csv", open it with notepad, then hit ctrl f, then type "TIER_KING;King;;;;;;;;;;X" into the new dialogue box.
Change it to this: "TIER_KING;Emperor;;;;;;;;;;X". That should be all you have to do. But again, this will make ALL Kings read as Emperor rather than King. You can also do this with Kingdoms in general, so they read as "Empire of Germany" rather than "Kingdom of Germany".