All the routes you create in Suunto app (see how to create a route here and how to generate a route from a workout here), import (learn here how to import routes from .gpx files to your Android device), or transfer via a partner service are conveniently stored in your account’s route library. From there you can edit them, share them, choose which one(s) to transfer to your watch, and delete them.
To access the route library, go to the map and then tap on the route library symbol on the bottom right. You will see all the routes in your library. You can scroll down to see all of them, share them or toggle them for use in the watch.
Tap on a route to see its details: un-toggle the route to remove it from the watch on the next sync, tap on the layers symbol to change the map layer, on the pencil to edit the route ad on the trash can to delete it altogether. After you are done changing the path of the route itself by dragging it in the map, tap “done” and you will be able to rename it, edit the suitable sport mode(s), activate turn-by-turn navigation, and toggle or un-toggle the “use in watch” option; tap “save” to keep the changes, or go back without saving. 
All the changes you make will be visible in your watch after the next sync.