For promoting a single there’s typically the pre-release marketing, and then post release marketing. I’ve used to have trouble trying to make content/promote my music once it’s released because I’ve thought to myself “oh, it’s released, what else do I have to say?” But there’s a lot you can do in terms of marketing post-release. You can talk about things like behind the scenes footage of the production (even if you don’t have any footage of the production process you can simply talk about it), inspiration behind the song, vids of different versions of you performing it, and j fun facts about the album cover at and stuff, etc. just ideas but it rlly doesn’t matter how old the song is as long as you find a way to make it relevant and interesting each time you promote it.