I just replace the whole thing if it has attic access. Have an electrician do it if you are not confident of your skill. Having the light work is not very helpful if the unit is shot. Clean and lubricate the fan shaft on your new one once a year and it will last a long time. You may be able to swap out just the fan if unplugs and it is not riveted in but you will have about the same time and money into it as replacement.