1) Get the sec_csc.zip from other device that have your CSC as DEFAULT csc
Download other device's firmware here, note that newer devices like Note 8 doesn't have CSC system. Best way is to use S7 Edge files. Android version doesn't matter.
After downloading, extract the zip file. You should have 4-5 files inside folder. Pick one that starting with HOME_CSC or CSC. It doesn't matter. If you have 1 file rom, pick that file. Change file extension from tar.md5 to tar. Open the file with your desired archive manager application. Copy "cache.img" or "cache.img.lz4" out of the archive depending on what you have. You now need to decompress lz4 file if you have lz4 file. Use this link if you have Windows. For other OS'es do your own research.
Now you got the cache.img. Extract the files inside it. These links might help to extract cache.img, if you have another OS, use tool named "simg2img" and a tool to mount or extract img file as ext4. I'm leaving these kitchen links to extract cache.img.
Get "sec_csc.zip" in the "recovery" folder, if you don't have this file, try extracting this from another ROM. Open the zip file, and take a look inside it. In the "system/csc" folder there's a file named "sales_code.dat". Open this file with text viewer, and check if this sales code matches with your desired one. If it doesn't match, try extracting sec_csc.zip with desired sales code from another ROM. Don't try to edit this file, if you don't have a custom recovery or if you don't want to experiment it as I didn't try it.
2) Get platform-tools (SKIP THIS if you will use SD-Card for installation. USE Only if you don't have SD-Card or don't want to install with SD-Card)
Download platform-tools from Google. You will use adb sideload to install the zip. Check 3rd step for further information.
3) Installation (READ THIS even if you know about flashing ZIPs or you will probably fail, this part is the most important part of this tutorial)
Reboot into recovery
Install zip (sec_csc.zip) using SD-Card or ADB-Sideload
Your device will reboot and change the sales code to default one in the sec_csc.zip that you flashed.
4) Reflash official ROM (ONLY IF your CSC is in one of the official ROMS for your device with your desired Android version) using ODIN.
I won't be explaining this in detail. If you don't know do your research. Just take 4-5 files rom and select AP BL CP CSC (i'd prefer to not select HOME_CSC) in the appropriate places. If you aren't expert make sure that you don't tick the Re-Partition, nand erase etc. and don't select PIT file while flashing.