[size=85%]If you're simply talking about updating a pre-existing submission, you simply go to the page of the skin, hit 'edit submission', and then the little pencil under actions to edit the skin file that's uploaded. If you want the skin to show up as 'updated x' you give it an update log and imput the update and hit 'save log' i believe- however this doesnt show as updated if the skin is a day or less old if my memory serves me correct

I hope this answers your question and helps ya out!
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What do you mean? Just put the skin into an image editor or use the skin editor here on PMC.