Apple Maps: Renaming Favorites or Marked Locations
To rename favorites in Maps on macOS (source):


Click on the magnifying glass in the search field
Click Favorites
Click "Edit" in the bottom right
Click on a Favorite's name to modify it
Press return or click out of the editing field, then click "Done"


To rename favorites in Maps on iOS:


Swipe up the "card" at the bottom that contains recent locations
Scroll to the bottom and tap on "Favorites"
Swipe from right to left across the favorite whose name you want to change, then tap "Edit Name"
Enter a new name, then tap "Save"


I've tried both of these methods on macOS 10.14.2 and iOS 12.1.2 and confirmed that they both work.