Due to the reboot, Sarah Connor has already become a fighter in 1984 in Terminator Genisys.
In serial fictions, the term reboot means to restart the whole series with a new continuity and discard previous settings. A partial reboot is a type of sequel that acknowledges some previous installments but discards others, and is meant as a follow-up to an installment other than the one that immediately preceded it. Additionally, a "soft" reboot is sometimes used to refer a sequel that acknowledges all previous installments, but discards part of the elements, including characters, from previous instalments.
In Terminator franchise, Terminator Genisys is a partial reboot, acknowledging the events of the first movie and some concepts from the second but also erasing them via time-travel while disregarding Terminator 3: Rise of the Machines and Terminator Salvation entirely.
The 2009 film Terminator Salvation poses as a soft reboot to the film Terminator 3: Rise of the Machines as Salvation removed the Cyber Research Systems introduced in Rise of the Machines while brought back Cyberdyne Systems which was claimed to be an "ancient history" in the third instalment.
The 2019 film Terminator: Dark Fate is also a partial reboot as it continues the story directly from Terminator 2: Judgment Day, disregarding the Terminator 3: Rise of the Machines and Terminator Salvation.