well sometimes the tip has a screw in it that stops it from roatating, look the whole flashhider over and see if theres anything that looks like a allen key might fit into, if there is grab a key and remove the little screw this should make it easier to remove.
your gun is a Classic army right?
apparently they use a rediculous glue on them.
boil water and dunk the tip in.
then un screw
as for installing the new metal flash hider its as easy as screwing it into place and then if theres one of those screws tightening that
"You know what the chain of command is? It's the chain I go get and beat you with 'til ya understand who's in ruttin' command here."