I need a help with Microsoft word. For some reason the word is 'talking' to me in American English! Z instead of S. I know I'm going to Florida next week but didn't expect the word to join in the fun too(!)

The word is shouting at me with red line underneath as I did not add z in the word. At times it does the flipping auto correction and I have to change it back with a S in it.

How do I change it back to British English?