I can't figure out how to flash the original stock firmware to my Samsung Galaxy S6. I want to download different version of Android on my mobile and I've found a useful firmware repository called sammobile but I don't know how to install the downloaded files on my mobile.
This is quite easy you have first to download the firmware files with the right model to your device (which you have done). Then you need to flash this files in Download mode using your PC with a tool called Odin. you can follow this guide for beginners with illustrated images and common problems Samsung Original Firmware Download and Flash Guide