The brush roller is usually located under the vacuum cleaner and some in the attachment tool. Usually you can fix a brush roller with a quick cleaning or by oiling it's bearings however once the bristles endure too much wear and tear, it is time to replace it.
If you are using your vacuum on a daily basis you will need to replace the brush roll much more frequently. You can take a look at your brush roll and see how the bristles look. If they look worn out and the bristles look loose it's time to change your brush roller. Monthly care of your brush roller will help to keep you vacuum working properly.