You take the thrust tube off and unscrew it, taking care with the electrodes.

Probably best to ignor what I wrote first time  

this is for the 3m so yes you take the control box off and slide the parts out.

A) Determine the proper firing rate for the boiler/furnace unit, then use Table B, page 9, to select proper nozzle size for the burner.
B) Remove the nozzle adapter, 2, from the drawer assembly by loosening screw, 1.
C) Insert the proper nozzle into the nozzle adapter, tighten securely.
D) Replace adapter, with nozzle installed, into drawer assembly and secure with screw,1


INSERTION OF DRAWER ASSEMBLY (see figure 13)
A) Loosen the screw 3, then unplug the control box. 4. by pulling it up and back at the same time.
B) Remove the air tube cover plate. 5, by loosening the retaining screw. 6.
C) Slide the complete drawer assembly into the combustion head. as shown. and secure it in place with screw, 7. Attach the fuel line to the pump.
E) Replace the air tube cover plate. 5. and carefully plug in the control box. 4. reversing step A.
D) Securely tighten all screws.