The nice thing about vaping is that there are few hard and fast rules. You do it YOUR way. Better'n Burger King!
You don't HAVE to clean out your tank, you can put a new flavor over an old one if you like. I've come up with interesting flavor combinations that way. Of course, if the flavors aren't compatible (Dragon Fruit over Bacon, or something like that, ewwww!), you'll want to clean your tank.
You can rinse it out with hot water if it's a weak flavor and have no trouble. Something strong will require more drastic measures. Me, I use a 2:1 solution of water and white vinegar for flavor changing. Some people use alcohol like pure grain alcohol (Everclear) or vodka, but white vinegar removes odors like nothing else IMHO. Soak your tank in the vinegar solution for maybe an hour, then rinse it very thoroughly. Dry burn the wick and rinse again, let dry. That should eliminate the chance of any flavor crossover.
Or you can do what many do, pick up a new tank each time you get a new flavor. Disposable CE4 tanks are usually 5 bucks at most vape shops and online, and a good way to get about trucking along with a fresh flavor labor free.