Apple Maps: Renaming Favorites or Marked Locations

To rename favorites in Maps on macOS (source):

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

To rename favorites in Maps on iOS:

1. Swipe up the "card" at the bottom that contains recent locations
2. Scroll to the bottom and tap on "Favorites"
3. Swipe from right to left across the favorite whose name you want to change, then tap "Edit Name"
4. 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.