If you cook regularly you will probably need to change the filters in your cooker hood every 6 -12 months in order to combat the build-up of grease.
Cooker hoods come in many different styles and shapes, but the principle is the same. If your cooker hood vents outside, then you will only need to clean or replace the grease filter, depending on whether it is a paper filter, or the newer style metal. Recirculating cooker hoods use an additional carbon filter, to cleanse the air before returning it to your kitchen. In this case the grease filters usually unclip from the bottom, and can be removed to expose the motor. The charcoal filter is clipped on to the side of the motor, and to remove it, you should turn it a few centimetres anti-clockwise, until you hear a click, and the opposite applies to install the new filter.
You can watch a video demonstration of this guide on the Spares Direct YouTube Channel.