From https://help.here.com/android/maps/#g275
Except when I do it (UK based) I just see "Routes" (not "Alt. Routes") . Selecting that seems to re-select the route I'm on, with no swipe left/right available.
To change your route while driving, swipe up from the bottom of the screen and select Alt. routes. Swipe right or left to see the alternative routes. To choose one, press Start on the desired route.
Except when I do it (UK based) I just see "Routes" (not "Alt. Routes") . Selecting that seems to re-select the route I'm on, with no swipe left/right available.