There is another way to change skin
but it works on Android only
(i don't know if this works on iOS)
1. using app - that manage files - go to the games/com.mojang/minecraftpe
2. open "option.txt"
3. change "game_skintypefull : Standard_Steve(or Alex)" on line 10 to "Standard_Custom"
4. and close that text file, go to the minecraftpe folder, put your skin file here and rename it to "custom.png"
5. now play minecraft!
me too
Delete the file and problem is solved
(In my case)