Advice on upgrading to iOS 6 from iOS 5 only I'm in need of updating the iOS on my device from 5 to 6 only (for test purposes). I already have iOS 7 on another device.

Does anyone know how this can be done?

The only option I can currently see is going to iOS 7 and then back down to 6. The problem I have is I don't want to find out it's not possible once iOS 7 is running.
Nope, once you update to iOS 7, you can't go back to iOS 6, since Apple stop signing the old software when the new software is released.

You can only install versions of iOS that are currently being signed by Apple for that device.

If your device can't update to iOS 7, then it is still possible to update to iOS 6 since the latest viable version of iOS continues to be signed for those devices.