This piece focuses on the increase use of smartphones of youths in South Korea.

Should I reword that phrase?

The only change I'd make is increased.

To reword the phrase "This piece focuses on the increase use of smartphones of youths in South Korea," you could consider the following alternatives:

- This article centers on the growing prevalence of smartphones among young people in South Korea.
- The main theme of this piece is the rising usage of smartphones among youths in South Korea.
- The primary focus of this article is the escalating adoption of smartphones by young individuals in South Korea.

When rewording a sentence, it's important to maintain clarity and coherence while expressing the same meaning in different words.