To replace a tip, scrape or cut off any remaining thread used to wrap the old tip. Sand down the rod tip for an inch or so just enough to roughen it up, then use ferrule cement to glue the new tip on. Ferrule cement comes in a stick and you melt it, dab it on the rod and then slip the tip over the glue and end of the rod. Make sure it is lined up correctly before the glue hardens.
An even more simple, effect way to get your rod tip fixed is to take it to a Sporting Goods. For just a few dollars they will put a new tip on your rod for you.
Rod tips tubes are measured in milimeters. You can order them by mm size, for example Bass Pro Shops catalog lists replacement Fuji tips from 5 to 8 mm in .5 mm increments.
The best bet is to take your rod to a local bait and tackle store and they can find one to fit. Berrys’s Sporting Goods in Griffin, my local store, offers replacemnt tip repair for about $10. The tops cost about $2.