Drain down and cap off the primary water pipes. Remove and disconnect the stove from the fireplace, clean the inside of the stove, remove ancillary parts such as grates log guard ash pan etc.
The boiler is removed and fitted through the base of the appliance. The complete ash pan and base unit is removed; the latter is attached with four bolts of 8 mm diameter (13 mm Spanner) two located at the front of the appliance below the log guard area approximately 8" apart, the remaining two are to the rear under the saddle boiler and symmetrical with the front pair. The Base plate assembly will have been fire cemented to seal the joint between stove body wrapper and the base assembly.
It is usually easier to turn the stove upside down. The base can then be removed.  A little assistance of a lump hammer may help.
The four 1" BSP boiler nuts on the rear of the stove will now be removed, and the boiler is prized away from the rear back plate. Having done this it can now be lifted clear of the stove body. (On the Larger units it may help to have assistance).
The Replacement/New boiler is passed through the base of the appliance and allowed to fall to the very bottom of the upturned stove body, the base assembly is now refitted (new bolts should be fitted) and secured using the four M8 bolts supplied. Having done this, the boiler can now be located in its designed holes, secure it with the four boiler lock nuts, ensuring the bottom two are tightened up first. The upper most two should be tightened up next (not forgetting to Fire cement the aperture under the boiler washers before fitting the lock nuts).

The Final part of the assembly is to amply seal the aperture around the stove body to the base plate with a generous "sausage" of Fire Cement.