The One UI 2.1 update is now available for Samsung Galaxy A51 owners. It comes with the April 2020 security patch but is missing a lot of features such as Single Take Mode, Pro Video Mode, Selfie Tone, and more. The firmware is available for download globally OTA and via Samsung Smart Switch.
https://www.notebookcheck.net/One-UI-2-1-update-for-Samsung-Galaxy-S51-now-rolling-out.464266.0.html