Verified answer

Equip different guns and you change all of the guns you have equipped, hip and chest holsters alike.

You buy guns at the store, but you can only equip one gun at any given time.

For example, you cant have the blunderbuss pistols as the first 2 and the golden flintlock pistols at the third and fourth.

You have to go buy them at stores, i think they will also replace the 1st and 2nd but still thats how you change them, like all of the assassins creed after the 1st