To rename a default system category,

1) Go to Company Settings
2) Select Categories
3) Select the category you want to rename and Click 'Edit'
4) Enter a 'Category name'
5) Enter a 'Category code' as per your companies accounting software.
6) Click Save to confirm.

For example: You can rename Transport (system category) to 'Fare' (customized category name) and assign it a category code 01.