Ok.. looking at the three metal lines where they head into the tank there is a metal retaining ring. It slides under part of the tank and has some tabs that stick up. You can tap those tabs counter clockwise with a screwdriver and hammer ...since no one owns the proper tool for the job.
Take a look at your new sender. It will have the sender, an o-ring and a new metal lock ring. That may give you a better understanding.
When you tap only one tab..the ring will move off center. Tap the next tab and so forth to center the ring and get it moving better. If the ring moves far enough off center use the screwdriver to recenter it and tap away. Wont take much to get it done, don't be afraid of it. You'll get it.