In order to delete several waypoints, i.e. a section of the route, you start by using the selection tool found at the bottom middle of the page (a dotted rectangle with a cursor in it). Using this you can select various waypoints to then remove them with the delete key or relocate them with your cursor. This is especially convenient when editing GPX tracks consisting of many waypoints.
In order to delete an individual waypoint, you can either choose the “X” next to the point or right click on the point and select “delete point”. You can also opt to “delete the entire geometry“.
Accidentally deleted a waypoint? No problem! Right-click on the point in the map you and select “restore” or to “undo” your action.