I think I can help with that. First lay the sax on its side with the bell keys facing up with the bow on your right side. Then locate the the small post (spring cradle) that the spring that keeps the low Bb key open attaches to. There should be only one for that key.
When you have found it, look to see what side the spring is on. It should be on the side that is facing you. If it is on the opposite side, take a small crochet hook (#9) and pull the spring so it is on the side facing you. Then try the key. The spring should just barely be strong enough to overcome gravity and keep the low Bb key open. If it does not, it needs adjustment.
To adjust the spring is easy. Unhook it from the cradle. Then with a pencil with the tip broken off or something similar, push the spring away from you to bend it back slightly. Don't go too far! Then try the spring again. Keep bending the spring back until it just keeps the key open. If you make a mistake and bend the spring too far and make it too stiff, then simply use your spring (crochet) hook and pull it toward you to take some of the curve out.
Good luck. A "real" spring hook can be purchased at musicmedic.com
John