Make sure you hold it until you get the 'saved' message... just a few seconds. I think it beeps also when saved, but it's been 3 months since I did mine, I I'm not sure about the beep.
It's in the manual, but short version... tune to the station you want, press and hold the button of a preset you don't want, and the new one will replace the old one.
I did try it today, and it cycles through the presets like the other poster said. What I did was set up my 4 favorite XM channels in a row, and then I can jump between them from the steering wheel. The screen changes within a second or so.
My voice command to change to another XM channel works fine, and the screen confirms the change in seconds. You can also hear the song abruptly change to something different.
Be careful what you wish for... I have a 2021 F150, and for the first year or so, most every time they did an update to fix something, they broke something else.
In fairness, the process was new to Ford at the time. Hopefully they have a better process now...
The SYNC 4 in my F150 allows this. Simply push the voice button and when it beeps ready, say "Channel 7" or whichever channel you want to switch to. TBH, I have not tried this in my Mustang yet. Give it a try, and I'll try to remember to try it next time I am out in it.
In my experience (Tesla and Ford), sometimes the OTA update returns some settings to default values. If something changes on your screen, you may want to check your settings and see if you can re-select the option you want.
It may also be a sign of a weak battery. The car will turn various things off to try to preserve the battery. I'm not sure if the radio is one of them, but it's worth mentioning. Do you have a battery charger/tender that you can put on it and see if it continues after the battery is charged?