need help with physics

Learning how to use Box2d...how do i do things like move_snap? Also when trying to create certain things shooting out form specific part of a physics object...for example how would one create flames from a rocket pack's jet? Everytime I try it doesn't come out from the specific point when it rotates.
Is there way to change the object's physic properties i set, during gameplay w.o fixtures or do i have to do the whole fixture stuff even though I've already set like the properties and modified collision shape?
 
I'd suggest making a physics object to spawn the flames from and join it to the player with a physics weld joint that way it will move and rotate with the player object
 
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