iPad 2 cannot instal iOS 6.1.3

Hello everyone. I have a question related to iPad 2 WiFi. Recently I got it from my friend and it was running very slowly on iOS 9. I looked up on IPSW.me and I found out that you can install iOS 6.1.3 on it, since it is still being signed by Apple. Furthermore, I found a video on YouTube describing the whole process and the uploader did the whole thing through iTunes without a single problem. To add up, the video was uploaded 2 days before I watched it. When I tried it myself, the iTunes simply refused the restoration saying that Firmware is not compatible. Anyone who did that here, can you please give me any advices on what to do? I'm new to all this. Also, forgive me for any grammar mistakes or misspellings, english is not my main language. Thank you.