How can one edit an existing sticker sheet

[Bertrude]

It's weird as I did this a while ago but can't for the life of me remember how I did it .

I opened up a sticker sheet in Photoshop (a single page with multiple stickers on it in columns and grids). It was the 'Loadus Nature Grass' sheet of stickers. I flipped it horizontally and (I think) saved it as png and added it to a duplicate sticker sheet of the original sheet to have a version that worked for the opposite direction.

I can't recall it being that difficult but can't remember what I did to get it into photoshop. If anyone has done something similar, please spare me from this self-induced frustration

EDIT: I've figured it out - the original files in the download of the Loadus Nature Brushes were all png files not stk. I must have edited one of those. Still, if there is a way to edit existing stickers sheets, I'd love to know it.