From an open source publication:
A missile's lower three propulsive cylinders are shipped first in a special trailer. Following routine maintenance, these stages are inserted into their new silo by a large vehicle called a transporter-erector. Next, the missile's two post-boost control system pieces are moved separately to Malmstrom for maintenance and reassembly. This package includes the rocket engine and guidance system-the missile's "brains."
Last to go is the warhead, which must undergo a week of disassembly and a week of reassembly before it's inserted anew. Once the warhead is in place, the entire missile system must be tested and programmed. On average, Malmstrom says its teams can remove and insert one missile a week. At about 50 missiles a year, that means Malmstrom's great switcheroo should be complete by late 1997.