Make the required changes on the logbook, in the sections provided. Note you'll need to provide the exact engine CC and engine number.

Post the updated V5 to:

DVLA
Swansea
SA99 1BA

Changes can take up to four weeks to return. Bear in mind they will quite likely ask for proof of the engine swap too, so you may need to take the car to a garage and have them write a note (on headed paper) to confirm the changes.