final Map<String, String> output =
           input.entrySet()
                .stream()
                .collect(Collectors.toMap(
                    o -> o.getKey(),
                    o -> o.getValue().getName()
                ));

Map<Integer, User> users = new HashMap<>();

users.put(1, new User());

Map<Integer, String> userIdToUserName = users.entrySet().stream()
        .collect(Collectors.toMap(Map.Entry::getKey, v -> v.getValue().getName()));