When you make something feel cleaner or newer, you freshen it. If your grandmother says she's going to freshen her makeup, she might apply some lipstick and powder her nose.
There are many ways to freshen, depending on what needs to be made fresh. You can freshen the air in your house by opening all the windows, or freshen up your friend's drink by pouring more lemonade in her glass. A company may decide to freshen their brand's image after a scandal, and you might freshen your breath by swishing a little mouthwash. Back in the 1600s, this verb meant "make stronger."