how can I change dates in Numbers from USA format to English format

Is Numbers already displaying USA format dates as true dates? (You can tell by first checking whether they are right-aligned by default, and also click on one to look lower left in the Numbers window to see if Actual is indicated.)

If they are displaying as true dates then you can get English format by going to File > Advanced > Language & Region and switching to Language: English and Region: United Kingdom and reformatting the cells as Date & Time in the Data Format dropdown in the Cell panel at the right.

If, on the other hand, they are text and Numbers does not recognize them as dates) and you are already set to the UK region, then try going the other way. File > Advanced > Language & Region and set Language: English and Region: United States. Then reformat as Date & Time, and switch back to your home region.

If you want to keep your machine in the US Region but use English format dates then you would need to adjust settings at System Preferences > Language & Region > Advanced > Dates.

SG