Go through all the folders and open all 3 .dds files (on any painting software like paint.net or firealpaca)
download an image you wish to use as the flag
copy the image onto the dds file, and if it asks if you if you want to expand canvas, press keep canvas size, position and rotate the image until it fits the pink box, then paste another image and do the same for the blue box (both boxes use same flag)
after you do it for all the files rename all files from .dds to .texture (ex. stn_prop_lobby_flag_df.dds => to stn_prop_lobby_flag_df.texture)
take the folder and put it in mod overrides
enjoy
If you have trouble finding the locations, the file path should look something like this:
Flag Template\units\payday2\props\bnk_prop_roof_flag_01\bnk_prop_roof_flag_df.dds
Flag Template\units\payday2\props\stn_prop_lobby_flag_fbi\stn_prop_lobby_flag_df.dds
Flag Template\units\world\props\bank\bank_flag\bank_flag_usa_df.dds
