Signature Raceware and Change Outfits in Umamusume. The guide includes how to unlock Signature Raceware, changing outfits, and a list of Raceware (3-Star Outfit).
How to Unlock Signature Racewear
Reach 3-Stars via Star Unlock

When you raise an Umamusume to 3-stars through Star Unlock, their Signature Racewear is unlocked. Star Unlock requires Star Pieces, which can be obtained from the Scout, Legend Races, or the Statue Exchange in the shop.
How to Change Outfits
1. Tap the Icon in the Upper Left of the Home screen

First, tap the icon in the upper left of the Home screen. The icon is located below your Team Rank.
2. Tap the Outfit you want to Change

Next, tap the outfit of the Umamusume you want to change. On this screen, you can also change the Umamusume that appears on the Home screen and in Races.
3. Select the Signature Racewear

Finally, by selecting the Signature Racewear you want to change to, you can change the outfit of the Umamusume that appears on the Home screen and in Races to their Signature Racewear.