Replacing guides is very straightforward. Just cut the old guide wraps off by cutting the thread on top of the guide feet. After the thread is cut, just unwrap it from the rod by picking it until you get an uncut thread and pull on it until all the thread is off that guide foot. Note, you must cut the wrap thread right on top of the guide feet or you will end up cutting into the blank, something you do not want to do because it will weaken the rod blank.
Putting new guides on the rod is also very straightforward. Get a spool of size A rod wrapping thread that matches the color already on the rod (or a different color if you want to change all of the guides or thread color). Tape the new guides on the rod with 1/4" masking tape and wrap the thread on the rod just in front of the guide feet, bring the thread down onto the guide foot (use your finger to help guide the thread over the guide foot and to keep the thread tight against the other thread wraps as you wrap over the guide foot), and when you are about half way onto the guide foot with the thread, put a loop of the rod wrapping thread or small diameter (like 5x or 6x) tippet on the bottom of the blank and wrap over it until you reach the guide's loop. the cut the thread while holding the thread wrap with your other hand's fingers, put the end of the cut thread through the loop, and pull the loop and thread through and out.
A good tutorial on this can be found in the rod building books on the market or on the rodbuildingforum.org site.
Replacing the tip's top is also very straightforward. If there is only a few inches broken, just get a new rod tip top that is the correct diameter to fit over it and glue it on with tip top or ferrule cement by heating the cement stick, rubbing some on the rod top, and immediately sliding the new tip top over it. Line up the new tip top with the other guides before the hot glue hardens and you're basically done. If simply replacing a broken tip top guide, heat it to remove it, add some more melted tip top glue to the rod's tip, and put the new tip top on.