To manage the folders go to More (o o o) | Various Account Settings | Tap your Account | Folder Mapping.
To change the folder mapping, simply tap the folder you wish to map, and select the folder you wish to map to.
In general, it should fit the folder your provider’s web client is putting emails into. This will guarantee your sync is correct: emails you archive/delete/send in TypeApp will be well placed for other clients to access, and emails you archive/delete/send in other clients will sync properly to TypeApp’s Archive/Trash/Sent folders.
Note: Folder mapping is applied only to IMAP and Exchange and not for POP3.