Small Edits to Product Packaging

Hi!

I need to make some small edits to some products packaging before getting it printed.

I have 20 different designs of product sleeves to wrap around boxes (all very similar apart from pictures and a couple of words); I just need to change the dimensions of these very slightly, to add 4.5mm to the front panel, and 4.5mm to the back panel. I have all of the designs available as GIMP (.xcf) files, and have included an example below!

Thanks in advance!