To rename a category please proceed as follows: 

• iOS, Android & Windows 11 / Amazon FireOS:
  Settings > Categories & Rules > swipe left on the main- or sub-category > Rename > enter the new name
  or
  Select the sub- or main category that should be renamed > Edit > change the name in  'category'
• macOS:
  Options > Categories & Rules > click right on the main- or subcategory you want to rename > Rename > enter the new name