In the episode 'The Cutie Re-Mark', which pony uses time travel to change the past?

In “The Cutie Re-Mark,” Starlight Glimmer uses time travel to change the past.

Starlight steals and modifies a time-travel spell created by Star Swirl the Bearded. She travels back to the moment when Rainbow Dash performs her first Sonic Rainboom, an event that helps Twilight Sparkle and her friends discover their cutie marks simultaneously. By preventing that event, Starlight tries to stop the friends from ever meeting.

Twilight Sparkle and Spike pursue her through the portal, but each alteration creates a different, damaged future. These alternate timelines include worlds ruled or devastated by figures such as King Sombra, Queen Chrysalis, Nightmare Moon, Lord Tirek, and Discord. The escalating consequences reveal that changing one crucial event can reshape all of Equestria.

Starlight’s motive comes from her childhood separation from Sunburst. After he earns his cutie mark and leaves to study magic, she concludes that cutie marks destroy friendships. Twilight eventually persuades her to abandon the revenge plan and give friendship another chance.

The episode is a two-part season-five finale, not a story about Sunset Shimmer or Trixie traveling through time. Twilight and Spike follow Starlight, but Starlight is the pony who initiates and repeatedly alters the past.