Firmware G998U1UES4AUJ7 is available OTA from most carriers for the US Unlocked S21 Ultra. It's 267 MB, and as the S (rather than U) before the bootloader number tells us, it's only a security update (Nov 1 security patch level). To see if your carrier has released it yet, visit your carrier's website or look at sammobile.com. If your carrier is late with it, you can download it directly from Samsung via Frija and install it via Odin.