[Samsung Galaxy S23 Ultra] xdauser2024:
You should post a full screenshot of your settings > about phone > software information page so people can provide you with more accurate/relevant info.

I'm assuming your model number is SM-S918U (US carrier locked). U1 is US carrier unlocked variant. U variant devices receive their updates from their carriers in the US, and not directly from Samsung.

What you'll need to do is to flash U1 XAA firmware (this is the latest one right now) on your U variant S23U. You'll need patched Odin because regular Odin can't flash U1 firmware on U variant devices.

To use patched Odin, you can follow these instructions or look up a tutorial on YouTube. The instructions to use regular and patched Odin are the same.

In your case, since you're flashing U1 on U, you should try to use CSC file instead of HOME_CSC one

It says in step 8 to use HOME_CSC file in the CSC slot in Odin. The difference between using CSC and HOME_CSC file is that, with CSC file, your device will be factory reset during the flash process, as in all your data will be deleted from your phone. With HOME_CSC file, your phone data is not deleted.

Since you're not only updating your firmware but also changing it from U (carrier locked) to U1 (carrier unlocked), it's best if you use CSC file during the flash process, which will ensure you won't experience software bugs later on.

Make sure to back up your phone's data before performing this procedure.

After flashing U1 on U, you'll be able to get OTA updates directly from Samsung, anywhere in the world.

And no, this process, when done right, won't void your warranty, trip your Knox bit, root or brick your device.

Edit:

This is a great tutorial I found on YouTube. It's for a S21U and it's by a Canadian user flashing unlocked firmware but this procedure is the exact same if you're flashing U1 on U on a US S23U.

At 5:41, he mentions that if your phone, while it's in download mode, is not detected by Odin, then you'll have to restart your PC and try again. This can happen if you've installed Samsung USB Driver for the first time, before using Odin. Because of this, it's better that after installing that Samsung driver, you restart your PC and also restart your phone. It'll save you potential troubleshooting headaches if you restart both devices before performing the flash procedure.

At 6:47, he starts talking about the importance of using the CSC file in CSC slot, instead of HOME_CSC file. If you use HOME_CSC file while you're changing your firmware from U to U1 then this (known as "Dirty Flashing") can cause bootloops and even soft-bricking your device. Don't take that unnecessary risk. Just back up your phone's data before flashing U1 firmware.

At 9:28, he mentions an important point about 'download mode' timing out. If you waste too much time while the phone is in download mode, then it's possible that your flash might fail and you'll have to restart this process. That's why, like he mentions in the video, it's better if you load the files in Odin before putting your phone in download mode. That way, as soon as Odin detects your S23U, you'll be able to start the flash process. This is an especially important tip for users with a slower CPU/HDD. The AP file can be 10+ GB. And when you load it in Odin, as you can see in the left hand log portion, what's actually happening is that Odin is verifying the file by checking its MD5 hash/checksum, to ensure that it's an official Samsung firmware file.