Hello i have downloaded some new backlighst for my XRT. I have put them to the DDS-folder, and then what to do now??

they should have been installed, more information?

If you have replaced a file, it should work already and should be shown in LFS on your XRT. There's nothing you have to do else than replacing a file.

I assume you are talking about XR_interior1.dds file right? Then you just put it in LFS\data\dds just make sure you've backed up the original files.