I recently got a Cayin N3 in a grab bag of MP3 players from Goodwill. The firmware upgrade is available from their google drive, but it’s a pain to actually get so here it is for anyone else looking for it. Cayin N3 Firmware v3.1 Just unzip it into the root of your micro SD card then go to Settings->System Upgrade

Official download page: https://en.cayin.cn/drive/9/18/567.html