So you will need to get access to one of the carnivores animal editors
Links to those can be found on the carnivores saga discord
Once you have the editor open you'll open the .CAR file of the rex, this will load it into the editor. From there; there should be an option for "texture" once that drops down there'll be an option to replace the texture just click that and then in your files you'll track down the skin you want to use and once selected it will appear on the model.
Then save it into your carnivores file
This process might be explained a bit easier in Saurian Target's demonstration tutorial
RaptorKlaw
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I upload carnivores content Spearhead of the Carnivores Ice Age Redux