[SDPhantom]

Assuming your :get() and :set() functions are working correctly, this is a simpler method.

["onclick"]=function(self)
    local newvalue=self.options[Wrap((tIndexOf(self.options,self:get()) or 0))+1,#self.options)];
    self.valueText:SetText(newvalue);
    self:set(newvalue);
end

Note: Both tIndexOf() and Wrap() are Blizzard-defined functions located in SharedXML\TableUtil.lua and SharedXML\MathUtil.lua respectively.