Water changes help keep your fish happy in multiple ways.
Fish live in their toilets. Their waste produces toxic compounds such as ammonia. If your tank is cycled, this won't be a problem, but you will end up with nitrate instead. Nitrates can be removed by plants and certain bacteria. This may lead you to wonder if water changes are required.
However, the water also becomes saturated with other chemicals such as allelochemicals and stunting hormones. That is not the only reason that you must do regular water changes - the balance of chemicals inside the water change as fish release waste and absorb use minerals. To sum everything up, you must do regular water changes to lower toxic chemicals and keep everything in balance.
You should do water changes every one or two weeks, possibly more, depending on how stocked your tank is. If your tank contains sensitive inhabitants such as shrimp, it is better to perform small, more frequent water changes to prevent large parameter swings. When performing a regular water changes, it is a good idea to remove around 40% of water in the tank. If you are performing many small water changes, target around 15% water changes. Unless you are keeping very high-bioload fish, or have an emergency such as a disease, never perform large water changes.
To perform a water change, use a water siphon or cups to drain water into a sink or bins and turn off electric appliances above the new water level. Then, fill up jugs or bins with water and dechlorinate it using a product such as Seachem Prime. Pour the water into the aquarium slowly, making sure not to hit fish.
Congratulations, you have completed a water change!