Type-in footer changes, in this case a date change and click “apply to all”.
Now that was simple.
And if it still won’t apply your changes, you may need to do as Roy does on The IT Crowd. “Have you tried turning off and on again?” I’ve found that for a few slides, the location or style is different than on the master, especially when you’ve dragged in slides from somewhere else. This is when I’ll do a reset on the footer (off and on again). Microsoft forgot to put a reset button.