Need some tips on necessary tools for editing skins

[Eerie]

Hi, sorry if i repeat questions asked elsewhere likely a thousand times, i tried search for information but was clearly not that good at it, didn't find what i'm looking for so i hope someone is so kind to answer my doubts here.

basically what i want to do is recoloring the skins of some units (example, make all orcs and trolls melee units a light gray/brown to populate a remote village in a corner of a map), reforged version.

i need to know what tools (possibly free, not very expensive ones like photoshop pls) i have to get in order to open the original skins, and edit those. can anyone explain me what i need to do in a quick list noob-proof way, for example "download 'this', install, run it, you'll find the skins of units more or less in this section of your wc3 reforged 'file name folder name', extract, edit like you wish, reimport doing 'this' and use for your new units"? thx in advance