Unfortunately we don't currently have the option to manually change the plant type after it has been added, but we're working on changing this in the future.

In the meantime, we'd recommend that if you want to have your plant listed as the correct type, you move the original, incorrect one to the graveyard (although we know this may feel a little morbid!) and then add a new plant under the correct type. 

You can even use a custom name such as 'Version 1' and 'Version 2.' This way, you will still be able to reference the original plant and see its history, but you won't get two sets of care reminders for the same plants.