The tube is easy to replace. There is a shielding sleeve you slide  
down, then pull the tube straight out. This is a little hard so watch your hands! If you have any oil on your hand you'll need to clean the tube and your hand with alcohol first. You may need to wiggle the tube a little to get it to come out easily.  
 
Once the tube is out, you can push the new one in and slide the  
sleeve over it. The only thing to be mindful is that some tubes are made too fat and the sleeve may not fit.