SpaceX upgraded the Merlin Rocket Engines
In 2015, SpaceX made a number of modifications to the existing Falcon 9 v1.1. A principal objective of the new design was to facilitate booster reusability for a larger range of missions, including delivery of large commsats to geosynchronous orbit. The design gained 1.2 meters of height, stretching to 70 meters including payload fairing.
Modifications included liquid oxygen subcooled to −206.7 °C and RP-1 cooled to −7 °C for density (allowing more fuel and oxidizer to be stored in a given tank volume), several size and volume changes to the first- and second-stage propellant tanks, and several small mass-reduction efforts.
Two key improvements were the replacement of the first-stage engine with the full-thrust variant of the Merlin 1D and the replacement of the second-stage engine with the Merlin Vacuum (1D).
The new first-stage engine featured an increased thrust-to-weight ratio while the new second-stage engine was optimized for higher performance in vacuum through modifications such as a larger exhaust nozzle and improved attitude control system.
First stage booster can reach low Earth orbit as a single-stage-to-orbit if it is not carrying the upper stage and a heavy satellite.