If you are browsing your user account's home folder in Finder and Library is hidden, press Command+Shift+. (that's a period) on the keyboard. All the hidden files in the folder will appear as translucent icons, including the Library folder. From there, you can double-click the Library folder icon to open it.

To change the icons of default libraries, you need to do a trick with their.library-ms files. To change icons of default libraries in Windows 10, you need to do the following. Open File Explorer and go to the following folder: c:UsersYOUR USER NAME[&AppDataRoamingMicrosoftWindowsLibraries&] In that folder, Windows 10 stores all Library files.