How do you assign weapons on assassins creed 2

I'll try to make it a little clearer for you...

For most of the game before you even get a weapon, even after you get to don an assassin's hooded robe, you have to fight with your fists. Once you have located a codex page with the plans of assassin weapon, pay a visit to your good friend Leonardo Da Vinci, have him decode the page and reconstruct your father's broken Hidden Blade. Once you find a weapon ship, you can buy more basic weapons that are only available through the progress of the story. With any new codex pages referring to Hidden Blade enhancements (Poison Blade, Double Blade and Gun), take them to Leo and he will construct them for you. You get to use all three all of the time based on how much poison or bullets you have.

In your inventory, you can fill the weapon slots with whatever is available at the time, but any new weapon you buy are replaced automatically, regardless of whether they are better or not. To outfit yourself with the best weapons available, visit your villa (once you claim it) and find the weapons and armor rooms to replace anything you prefer. Any blunt weapon replaces the sword and vice versa.

Visit a shop the moment you enter a new city to see if a new item is unlocked and keep following the story. It is pointless to attempt to find unavailable stronger weapons while current enemies wielding swords are barely skilled enough to take out one man with a dagger. Once you've developed the skill, you can temporarily "borrow" a weapon from an attacking foe by disarming him. Unfortunately, you don't get to own them because you have to drop them eventually and you can only take them while performing a counter while barehanded (fists).

There you go... clear as mud.