I read this book as a kid, so that would have been between 2010-2016.
The female character travels to more places than just the World's Fair, but that was the most action-packed travel. This book may have been part of a series, I'm unsure. I recall that whatever tool used was blue and/or maybe a medallion.
I think it was a YA book.
A Google search turns up Timebound (2013) by Rysa Walker.
The blurb reads:
When Kate Pierce-Keller's grandmother gives her a strange blue medallion and speaks of time travel, sixteen-year-old Kate assumes the old woman is delusional. But it all becomes horrifyingly real when a murder in the past destroys the foundation of Kate's present-day life. Suddenly, that medallion is the only thing protecting Kate from blinking out of existence.
Kate learns that the 1893 killing is part of something much more sinister, and her genetic ability to time travel makes Kate the only one who can fix the future. Risking everything, she travels back in time to the Chicago World's Fair to try to prevent the murder and the chain of events that follows.
Changing the timeline comes with a personal cost--if Kate succeeds, the boy she loves will have no memory of her existence. And regardless of her motives, does Kate have the right to manipulate the fate of the entire world?
It's the first book of "The Chronos Files," a series of 3 novels and 3 novellas.
