How to use UpdateAsync on a Table?

So I have a table which can be added new indexes. Now I am confused and don't know how to save those new indexes. My idea was to override those new indexes, no idea how. Any suggestions?

local success, Error = pcall(function()
	powerDataStore:UpdateAsync("AvailablePowerData-"..player.UserId, function(oldData)
		local saveTable = oldData or {}

		for _, v in pairs(powerTable) do
			-- i stopped here, dunno how 
		end

	end)
end)