1) Install Photoshop, then simply install the plugin from "1)" folder (or install standalone app from this
website: https://developer.nvidia.com/texture-tools-exporter)
2) Select any .rpack file you'd like to edit, to make your choice easier, simply drop random .rpack file on
.exe file in the folder "2"
3) Then choose the model/texture (not sure about models but ok) you'd like to edit and don't delete it (the
same thing with .rpack file you got this texture from)
4) Edit this texture by using the photoshop plugin from step 1, save it as nvidia's dds and use the setting
from the image i left in the folder of step 1
5) Drag the same .rpack file as the textures one you'd like to edit and copy it to the folder "3", then,
run "quickbms.exe" and when it'll ask you to select the file, do it in this order "dying_light.bms"->".rpack
file"->"folder, where you'll let the script to extract info from rpack files", then you'll get some .dat files
or others, in this folder, some of files are useful, some are usless, determine it by yourself somehow (or try
to do the step 6)
6) Run the executable from folder "4" and do what it says

7) Explanation of step 6: The .dds files which u get from step 2 is a game textures, and as i know by far, there
is currently no other program to pack edited .dds textures back to .rpack files, so here comes QuickBMS, this
program can make .dat(and other) files with the script we used before, and after some research, i found out
that ONLY this .dat files have the same binary code parts at some parts of its binary code (sounds goofy), so,
what we're going to do is to select .dat file in the last program from step 6, so the program knows which
file to edit, then we're going to select ORIGINAL .dds files we got from step 2, then this program is going
to find exact matches in its binary code so it knows what to replace, and then we're going to add edited .dds(.dds1)
files and then start the process of Applying changes to .dat file, if you did everything right, it's going
to ask you where to save the output file and'll say you that the process is done, and if it gives you an error,
try to read steps and explanation again, or try to figure it out by yourself, because it MUST WORK.

8) Drag the edited .dat file to the folder you selected as the folder to extract .dat files in the step 5 and
replace them with edited .dat file (new files must have the same name), then, if it's all you wanted to do with
this rpack file, simply delete any other .dat file from this folder (so the packing process can go faster), and
then run reimport.bat in the folder of step 5 and redo every step you did in the step 5 the same way (by
selecting .rpack file, you're saying the program which file to edit with .dat files from the folder you're
about to select), it must repack it without any problems (if it says that the input files are larger than
original, select to replace it anyway, it doesn't ruin anything as much as i know), then, if it has finished
the process, simply close the the console and drag (MAKE A COPY OF THE ORIGINAL FILE) the edited .RPACK file to
the folder of Dying Light where you get it from and try to launch the game...
________________________________________________________________________
Troubleshooting:
1) Sometimes, if you're replacing only one texture or adding only some small detail to it, you'll need to find
in that .rpack file some dds's with transparent spaces (like textures with Alpha channel), and add rgb noise
behind parts that is not visible, it'll make .dat's file data to be flooded with more info, and somehow it does
magic...
2) If you're having problems with program from the folder 4, idk, works for me but ok, the best i can say is
that .dds1 file MUST have the same name as the .dds you chose as the original (that's how my code works) 
3) Currently, because of QuickBms's memory allocation restrictions it isn't possible to edit files that weight
is around several gigs or more (PLEASE, SOMEONE, I NEED HELP WITH THIS PROBLEM, WE NEED TO REWRITE QUICKBMS SMHW)
________________________________________________________________________
Credits to:
Parapando, the author of this guide and one of the tools (the last tool)
QuickBms tool author - Luigi Auriemma
DDS texture extractor tool - id-daemon (probably, i'm not sure)
Nvidia plugin - Nvidia