or do you mean....
….that skinny line built into the sail “luff” to hold it onto the furling gizmo?See, the line that makes the sail go up is the Headsail Halyard.The skinny one that feeds into the groove is a luff tape, sometimes called a luff line.Now, assuming you mean the “halyard” you need to get a line as thick and strong that stretches little to none. If your old halyard is still on there, tape the new and old ones together and carefully pull it through. If the mast is off the boat, it’s a simple matter of feeding the new one. At the top of the mast, there’s these roller like things called “sheeves” make sure they turn freely.They have good prices on quality lines and pre-made halyards on this very site.If you mean the luff tape, you’re gonna need a sail maker to put a new one in.So, register on this site, it’s free and very handy. Do the owner’s profile so people know more about your boat and your skill & expertise.Best of luck, keep us posted, eh!Ferg