The firmware update is OTA now for most carriers. To see if your carrier has released it yet, check your carrier’s website or sammobile.com. If your carrier has not released it yet (or if your phone is not receiving it), you can download it via Frija and flash it via Odin (Google the instructions).
G99xU1UES4BULE is out now for the US unlocked S21, S21+, and S21 Ultra. It has the Jan 1 security patch level, and as the S (rather than U) before the bootloader number tells us, it is only a security update (no feature, stability, or performance improvements).