My friend made her Minecraft account when she was around 8, she's now 11. I don't know for sure if the skin she is stuck with was her first ever after default Steve. Still, after uploading her custom skin, she couldn't change it. Does this have to do with how old her account is? The email?
Did you make sure to hit "Save" under save and upload? That could be why, or maybe the skin file is too large/small. If you need, use Skin Craft becuase it is an easy way to make skins, and will automatically size them for you.
This had happened to me before. I had changed my skin but it showed a steve skin. This problem might be that you have the skin on but in your view it looks like a steve. Luckily I had another account for minecraft. in its view, it showed my skin. If you want more detail, please put more information like what did you do? What skin did you upload? Try using this website. https://minecraft.novaskin.me/