Click onto your profile picture on the top right, head to settings. From there, go to "channel and videos", and on the left hand side under the settings tab again it'll say "stream, channel, moderation". Click on stream and you'll have the option to turn on "store past broadcasts", BUT when when the stream ends, the VOD lasts for 7 days and then you have to highlight it for it to stay however long you want it on your channel.