Update image in atlas

Hi
I'm revisiting an old game built in Unity 4.6 and need to update a few UI images in an Atlas, but for the life of me can't remember or work out how to update individual images within an atlas, and a trawl through the web and tutorial videos hasn't helped so far.

In Sprite Packer none of my atlases show up (ie View Atlas has no entries) and all of the images that are embedded in my atlases are set to TextureType: Default, TextureShape: 2D. None of the images are set to Sprites so none have Packing Tags so don't even know what the Packing tag was for each atlas i have. I'm now using Unity 5.6 but opening an old project in 4.6 looks much the same?

I don't want to create any new atlases - just to update a handful of images that are contained within some of the atlases. Could anyone point is in the right direction about how I go about this. I'm sure it was a straightforward process - a year out from coding has just wiped it from my memory.

Thanks