Help!

You click "update this skin" and a new site shows up. You should see the 2d view of your skin there, to the right are 3 icons. Click the first one ("edit media"), a new panel should show up. Now you can upload the new skin file, and hit "Save", then "Save again at the top right.

As WolfOfDoom said, it can take a while until the skin preview view is updated.

GarfieldenI Updated My Skin And It Cant Update My Skin But I Can Update Details Dont Worry
If U See My Post PlanetMinecraft To Fix It I Will Be Happy

So you updated the skin, but you can't update it? I'm sorry, and I mean no offense. I'm just having a hard time following.. The preview can take some time to update, by the way..