This is somewhat pseudo code, but you can do something like this.
var bullet_tex1 = preload("res://bullet1.tex")
var bullet_tex2 = preload("res://bullet2.tex")
var bullet_tex3 = preload("res://bullet3.tex")
onready var bullet_sprite = get_node("bullet_sprite_node")
func switch_texture(num):
if (num == 1):
bullet_sprite.set_texture(bullet_tex1)
elif(num == 2):
bullet_sprite.set_texture(bullet_tex2)
else:
bullet_sprite.set_texture(bullet_tex3)