Urgent help please

[Fluffy Lambs]

That I have not had to do much of so I don't really know the best way; you wouldn't want to just wait till it dies but I guess you could try snapping its neck maybe?

 

[Nabiki]

I'm sorry that you have a chick that is apparently failing to thrive. Culling a chick is so hard emotionally, but very quick and easy physically. A sharp pair of scissors will take the head off for a very quick death.

 

[FloorCandy]

It could be a ruptured air sac:
https://www.backyardchickens.com/threads/air-sack-rupture.1175586/