The formula =PROPER(A1) will make every first letter of each word upper case
and the rest of each word lower case. A1 is the cell you want changed to
upper/lower.
So QUICK BROWN FOX becomes Quick Brown Fox.
If you wanted "Quick brown fox," that cannot be done via formula. A period
may or may not be the end of a sentence, and certain words in the middle of a
sentence may need to be capitalized, so this is just too complicated. Excel
can do =UPPER, =LOWER, or =PROPER.