shut valve off to filter loosen compression nut on valve and pull plastic hose out. where copper line goes into filter it should be a push fitting. push down on plastic around copper with needle nose pliers and pull copper out. its easier with someone to help. once that is out pull copper up to valve slide new 1/4 inch compression nut on copper then new 1/4 inch ferrell . push copper into valve slide nut and ferrel down and tighten making sure copper is pushed in all the way. tighten till you get resistance then 1/4 turn, you can always tighten more if you have to . turn water on and test. all ice maker lines i have seen are 1/4 inch.