Follow these steps to change your skin. After you design a skin in Tynker, you can select "Use" to download it.
1) Click the "Use" button
This will download a file to your downloads folder on your computer.
2) Log in to Minecraft
Visit https://minecraft.net on your web browser. The click on the "LOG IN" link.
3) Enter your Minecraft username and password
4) Edit your Profile
After logging in, click on the "Profile" link on the top of the page to go to your profiles page where you can upload a new skin.
5) Upload your Skin
Choose the "Steve" or "Alex" character you want your skin to be uploaded to. Click the "Choose File" button and select the file you downloaded. Then click "Upload".
Your skin will be updated in a few moments.