You will need an outside recording app to record it in a video format, and then you can upload it to youtube. Rivals replays are not recorded, but played out "live" with the same button presses to recreate the battle. As such, it's not technically a video, but more a reenactment. I've used mobizen as a free screen recording app, but the sound is pretty bad on it, perhaps with some fiddling I could get the sound better.