The process of converting from a LAPL(A) to a PPL(A) is called PPL (A) crediting.

If you hold a LAPL (A) and are applying for a PPL (A) you must have complete the following:

At least 15 hours of flight time on aeroplanes after the issue of the LAPL (A), of which at least 10 hours should be flight instruction at an ATO.
This flight instruction must include 4 hours of supervised solo flight, of which at least 2 hours must be solo cross country, with at least 1 cross country flight of 270km (150NM) or more, plus full stop landings at 2 aerodromes different from the departure aerodrome.

For the definitive information on converting please check out the CAA web site here