Well that's easy.
You can do this:
local skins = {1,2,3}
--"nextSkin" is the created button in your window.
function setSkin(button)
local i = 1
if button == "left" then --if you left clicked the button.
if i > #skins then --if it's bigger than the total skin number
i= 1 -- set it back to 1.
--do whatever u want here.
else
i = i+1 -- if i is not bigger than the table amount, then increment it.
end
end
addEventHandler("onClientGUIClick",nextSkin,setSkin,false)