Change object using script

I made an alpha menu for my game, but i don't want that in the final version of it. It was pretty simple, you had buttons, like: NEW GAME, OPTIONS, EXIT, etc... As it is a FPS, you need to select your weapon, and i am stuck in that part.

I made a menu that when you select the weapon selection, the game brings the player to a scene where there is a character in the center of the screen with a weapon and he is playing an animation. In his sides, there are two arrows that indicate that you can change to another weapon.

I wanted to make a script that would do the following:

The PLAYER click on lets say the right arrow. The game will CHANGE the character with a SHOTGUN prefab to the character with a MACHINEGUN prefab, and the other one with the SHOTGUN will DISAPPEAR before it. The PLAYER click on the LEFT ARROW, the game CHANGE the character with the MACHINEGUN BACK to the one with the SHOTGUN.

Here is an image of my menu: Image Link

I would really appreciate if anyone could help me, because this and the enemy AI are the only things that i need to finish before releasing my first game =)