On my machine I can find three ways to change to upper case. Each has its advantages and disadvantages.

If you have just a few cells to change, then Edit Transformations > Make Upper Case should do the job. You have to do it cell-by-cell.
If you have a lot of cells to change, then you can select them and use Format > Font Capitalization > All Caps. That will change how they display. But note that the actual value in cell remains unchanged. That is fine if all you want to do is display. But if you export you will not get all caps the way you might be hoping.
Another option is to use the UPPER function. That can also transform a range without going cell-by-cell, though you will have to clean up afterward by selecting the cells with the results, command-c to copy, Edit > Paste Formula Results to remove the formulas.
UPPER - Apple Support
The Capitals font doesn't seem to be included on my System (Sonoma 14.2.1) so I wasn't able to test that. Maybe Jerry can show how to get that font as it sounds useful.
There are probably ways to use styles to control capitalization in your tables but I've never gotten very far with styles in Numbers.
SG