Add a Sprite for each of the weapons to your player scene. It's OK if they overlap, because you're going to only make one of them visible at a time, using the
First, have a variable to track which weapon:
var current_weapon = 1
Use input to switch it:
current_weapon = 1
# you can also trigger animations or play sound effects here
current_weapon = 2
Then whenever the player uses the weapon (keypress, autofire - however you have it implemented), call the appropriate function that fires a bullet, plays an animation, etc.