Mods do not usually contain tracks; only cars. To install new cars, you can use CSM, which installs a 'carset' (xxx.cuh), or you can install them one by one.
To change the tracks you can either use a trackpack (xxx.tuh), which has all the tracks used in a season, or you can install the tracks one by one.
There are docs in the
FAQ section telling you how to do all these things.