Complex sentence: The engines themselves will be upgraded to the more powerful Merlin 1D, increasing payload capability from about 9,000 kg to over 13,000 kg. The new first stage will also be used as side boosters on Falcon Heavy.
Split into simpler sentences: The engines themselves will be upgraded to the more powerful Merlin 1D. All this will upgrade the payload capability from about 9,000 kg to over 13,000 kg. The new first stage will also be used as side boosters on Falcon Heavy.