replacing values of a matrix with an if operation using matlab

    mn=1 
    for kn=1:199 
      for sn=1:19773 
        if abs((x1c{kn+1,1}(sn))-(x1c{kn,1}(sn)))>=20 
        extract{mn}=x1c{kn+1,1}(sn); 
    mn=mn+1;
    end
      end
        end
extend=cell2mat(extract)+40;

How can I change the values of "x1c" with the values of "extend"?