If the filter hasn't been cleaned for a while, then it won't hurt to clean it.
Get a bucket of tank water and squeeze the filter pad out in the bucket of tank water. Do this for a minute or so until it is clean. Then put the pad in the aquarium for the time being.
Wash the filter case under tap water.
Wash the impellor (magnet with 3 or 4 plastic blades) and inside of the motor where the impellor sits. Use tap water for this. Watch out for small plastic or rubber washers that might be on the ends of the impellor shaft. The impellor shaft is a steel rod that goes through the middle of the impellor.
When it has all been cleaned, put it back together on the side of the tank. Then put the filter pad in and fill it with aquarium water. Then turn it on.
Filters should be cleaned at least once a month.