Please note that there is no way to restore lost encryption password!

You can't encrypt individual notes, but you can encrypt any notebook with all its notes when you create it.
You can't change notebook password, but you can create a new notebook with the new password and drag all notes from your old notebook into it.
When you encrypt the notebook, you also encrypt all files that are associated with it: embedded files and images, its history of changes and all future backups. Previous backups will remain unencrypted, therefore it's a good idea to download and delete them from the server.