If you mean the XR_Interior1.dds, as in the lights, you'll require a good graphical editing software, (e.g. Photoshop, Paint Shop Pro, Paint.NET, GIMP, etc.) and if that software doesn't save into Direct Draw Surface (.dds), then there should be a dds converter at LFS Database anyway.
To get started, you'll need to know exactly what look you want to achieve when each part of the lights illuminate.
There are various zones in which the dds is used for the lights (e.g. There's brake light zone, reverse light zone, and a turn signal light zone).
The XR's rear lights should be the simplest to modify, as there is a large area for the brake light zone and the turn and reverse zones are just to the sides.
Be mindful of where you get your image sources from, as you'll have to edit those properly to make your modifications at least decent.
