Can't change skin.

Heya Mojang, so uh, I bought Minecraft Java, and after I did, l downloaded a skin, put it on, and then I made a world in the game. It didn't let me use it. I was Alex for some reason. I tried to restart my computer and relaunch the game, but it wouldn't let me use my skin. So I'm stuck as Alex for some reason. Please help. (I'm using a Microsoft account)