This article contains some spoilers for Star Wars: The Rise of Skywalker.
Actress and Carrie Fisher’s daughter Billie Lourd portrayed young Leia in that scene in Rise of the Skywalker.
According to Yahoo Entertainment, there was a decision made to not replace Fisher with another actress, so instead almost all of Leia’s scenes in the movie are made up of VFX efforts and old footage. The only exception is one scene in which we see a young Luke Skywalker training his sister and showing her Force sensitive nature. During this sequence, Leia is played by Billie Lourd.
Source: IGN.com Billie Lourd Portrayed Carrie Fisher in Star Wars: The Rise of Skywalker Scene