Not sure but I assume its the same as the F18, which go here:
C:\Users\YOURNAME\Saved Games\DCS.openbeta\Liveries\FA-18C_hornet\Whatever you want to call the skin. Example
C:\Users\YOURNAME\Saved Games\DCS.openbeta\Liveries\FA-18C_hornet\RCAF - Bonedust
You can have many many skins by putting them in their own folder as above and changing the description.lua file at the bottom of the script to reflect where the files are example:
name = "RCAF - Bonedust"
countries = {"USA",}