How to move body parts and weld it (Is there a easy way to do it)?

I'm trying to move the lower torso on character but it comes back to old position how can i fix this?

I have a StarterCharacter and i will make a first person leg looking so i need to edit the legs.. im editing it but when i move the lower torso up and put it on starterplayer and test it it comes back to the first position