steps:
1. select destination
2. select navigate
3. select alt destination
4. select navigate
user must "stop navigation" first, app does not update and does not indicate this (only obvious if beginning of route is different)
(using android app)
ps. doesn't sound like a huge deal but I was almost crushed to death (my car is now scrap metal) after passing my alternate destination and attempting to reverse in the dark on a narrow road.