Template for replacing flag stickers. Open the .psd file in Photoshop or Photopea (free)
Path for flag stickers if you need it: PAYDAY3/Content/DLCs/00010-DLCHEIST0002/Gameplay/Weapons/Stickers/Flags

Note that this is not a template for changing what appears in the vendor shop as that is pretty easy.
Path for that: PAYDAY3/Content/DLCs/00010-DLCHEIST0002/UI/SS_UI_Inventory_Stickers_00010-DLCHEIST0002_flags.uasset

Video:

payday 3 modding is painful