how do I retrieve a text message accidentally deleted?

There are a couple of options. The first one is to restore your phone and use the last backup, given the fact you actually backup your iPhone and haven't backed it up since you deleted those message. The other option is to download an application onto your computer that can preform this actions. There are a lot of applications now that can help restore those deleted messages, but some cost money. If not mistaken iFunBox can do this and there are others as well. Best of luck!

There is iBackupviewer amongst many other apps.

http://www.imactools.com/iphonebackupviewer/

If you have a iPhone backup from before you deleted the message: do not connect the iPhone, start iBackupviewer and select the iPhone and then click to read the backup.