In the blue section of the log book (section 7 IIRC) just enter the new engine number and cylinder capacity then sign section 8 and then post the log book to the DVLA.

Think there is also something in section 7 where you have to enter the date from which the changes occurred as well

Also remember you will need to re-tax the car if you have changed from an engine less than 1549cc to an engine larger than 1549 cc