First, you need to open the Preview for Developers App, and uncheck the box. Then, you may need to downgrade your OS.
The only way to return to a commercial non DP version of the OS is by downgrading the phone through the
Nokia Software Recovery Tool. However, before doing so (which erases all your data from your phone), you may want to check that you are indeed running a version which is not available to those who are not under the developer preview program (since you mentioned that you have not been able to keep yourself up to date with WP, and may have not updated to the latest DP versions). Check that your OS version is not newer than what is posted in the
Windows Phone 8 Update History page. If it is, then you have a newer DP version and you can downgrade. However, if it is the same, then you won't be able to downgrade into anything, since NSRT will update your phone to what is posted on that page.
If you have something older than what is posted, then you may want to update without being enrolled in the DevPrev Program.