Each new build is auto-saved at level 0 (creation screen), level 1 and level 24 and it can be loaded at that point. Moreover, this feature keeps track of the last three level-ups, so you can load (revert) up to three levels back.
Click on the gear icon in Actions panel.
(https://i.imgur.com/lIoehm1.png)
Choose which save to load.
(https://i.imgur.com/GxwtX2E.png)
For now, this feature is disabled on character creation screen.