Yes, you can update your Single Issue App by following the same instructions for updating any type of Mag+ app. Just remember that, per our recommendation, you'll want to get your update submitted before January 29 to avoid Apple's February 1 cutoff date for Mag+ 4.4 apps, that way your app will remain available on iOS5.1.1 and later. See our announcement on this below:
If you currently have a live Mag+ app but you have not submitted an update since Nov 19, 2013, we strongly recommend you build and submit an app update prior to January 29, 2014. This will take your app to v4.4, the last Mag+ version that will run on iOS5.1.1 and later. After Feb 1, Apple will not accept a Mag+ 4.4 or earlier app build. (If you have submitted a 4.4 app prior to Feb 1, but it has not been approved by Feb 1, Apple will still approve it. You just can’t submit a 4.4 app after Feb 1.) There are critical bug fixes in v4.4, so building and submitting an update now will ensure that all your non-iOS7 customers have the latest code version.
Here is a tutorial on updating your iOS app.
Mag+ Support Team