If the filter won't unscrew it is probably so choked up with rubbish that the rubbish is catching on something and stopping it from turning. If it was me I would tip machine back against wall, get under and remove the pump from the other side. Not an easy job for a novice. Either that or force the filter and break it. Then buy a new filter and clean it out more often. Are you sure that the filter is leaking. Have you actually looked and found the source of the leak (where the water starts from)If you can't actually see it leak, I would leave it alone for now and look elsewhere. Sometimes it is a small spit or pin hole in the rubber door seal.
Lincoln Appliance NZ, January 2018