First you have to place the Lamy nib on a hard flat surface like a table. To protect the table for dents caused by the 2 small L shaped metal nib profiles resting on the table it is a good idea to put some paper between the table and the nib. Find a rectangular straight piece of wood of roughly 20 to 25 cm (8 to 10 in) length. Then carefully rest the piece of wood on the top of the rear part of the Lamy nib where the Lamy logo and nib type designation are written. The wood should only cover the rear part up to the breather hole or the nib size designation in case of a stub nib. Beware to keep away from the front end of the nib where the breather hole and/or nib slit is! Now whist keeping the piece of wood parallel to the table carefully apply some downward force with your hands on the piece of wood. This will slightly deform the rear part of the Lamy nib that is used for the feed connection making it fit more tightly on the feed. Never press on the nib with great force to avoid excessive deformation! Instead gently apply pressure in increasing small increments whilst regularly checking if the rear part of the Lamy nib got deformed enough to appropriately fit on the feed. If a Lamy nib is very tightly fitted on its feed and hard to remove by hand you can resort to grabbing the nib with a piece of rubber for extra grip. A more drastic method to remove an excessive tightly fitted Lamy nib is using sticking sticky tape stuck on the top of the nib to pull the nib off its feed.
Source: http://www.fountainpennetwork.com/forum/topic/121161-lamy-loose-nib/