You cant find the skin folder on Osu!Lazer or maybe you can but its just too complicated for non-brainers like me XD.
But yea if you wanted to modify the skin folder but you are unable to find the skin folder and chose to redownload but instead booted up osu!lazer which makes you unable to find the skin here is the solution for it:
First Download the skin / Export the skin to .osk
After that right click the .osk file and open it with osu! (Try the first or second osu to see which is osu! and which is osu!lazer)
Then go Windows + R %appdata% -> Local -> osu! -> Skins
There you go! Your skin is there and you can modify it if you wanted
Sorry if my English is bad or something, I tried my best to make it as understandable as it can.
and to get your skin onto osu!lazer
redo 1-2 but this time you need to find the osu!lazer when you are using open with instead of finding the normal osu!