To rename a variable, method, parameter, class etc. position the caret at the symbol you want to rename and press Shift+F6.
An in-place editor opens. IntelliJ IDEA will automatically replace all the occurrences as you type.
Alternatively you can open the Refactor menu and select Rename.
If you rename a class in a file with same name, the file is also renamed; the same is true for companion objects---they are renamed as well.
Renaming of apply methods is not always safe and may break your code.