To remove a trip, from the home screen, tap the options button °°° at the top right of the picture then select Delete Trip.
From the home screen, tapping the options button °°° will let you duplicate a previous trip to re-use the exact same packing list. The list will still be updated based on current weather to add or remove items that aren't necessary, an umbrella for example.
If the trip was created as a single-destination trip, it currently isn't possible to change it to a multi-destination trip. This feature will be available in the future.
To create a new multi-destination trip, from the home screen, tap on Add Trip, then select Multi-Destination from the selector at the top.