This heavily depends on what the changes are that you need to make to the LUT. If its e.g. just the polarity of a signal, you can use a XOR + control register. You can also use a LUT with 4 or 5 inputs, and use the extra ones to create a different logic.
I want to be able to completely change the LUT. There is no logic that I can apply to do this. It depends on values I get from a sensor.
No easy way to perform what you want to. There are a bunch of "registers" that perform the behaving of an LUT, but that is rather complex and not easy to understand. Have a look into the "PSoC® 5LP Registers TRM (Technical Reference Manual)".