Whenever I want to change into a custom skin I have to close and reopen Minecraft.
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Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-37926.
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I am having a similar issue with Windows 10 as well. I can open and search for the file, but when it shows me which type I want to choose (Alex/Steve) it is the same skin. Then, when I reopen the game, the new skin suddenly is there!!!
I recorded a small video to show you the process, and can upload it and share the link to you if you wish, since it is too large to upload directly here...