The way smart objects work is you have 1 smart object which gets propagated many times. Essentially "clones" are created of the original Smart Object. When you edit the original or any clone... all clones will update to show edits.
To make each box different you'll need to deconstruct the mockup and create additional, independent, smart object layers for each box. Basically, you'll need to deconstruct the "link" a smart object has with all its "clones"
You can do this via the Layer's Panel menu - highlight a Smart Object layer and choose
New Smart Object via Copy. You'll then need to reposition this new SO layer properly and remove the existing SO layer for that particular box... and then repeat for all other boxes which are to be different.
There's no "break link to original smart object" command that I'm aware of. Only the
New Smart Object via Copy will duplicate, then break the link to the original smart object.
There's no easy, simple, one or two step process for this.
The point of that mock up is that all the boxes are the same. So, it would appear to be constructed to facilitate that aspect as one would expect.
The other option would be to rasterize the smart object layers converting them to standard, unlinked, raster layers you could then edit independently of each other. But with that, you'd need to transform each of these raster layers to match the perspective of each box again.