Shower her with candy lmao (I'm kidding, don't (unless if she likes candy and you have candy to give to her)). Idk, whenever my gf is upset I always tell her how much I love her and how amazing she is, and that she doesn't deserve anything bad that happens to her. If she's over at my house I'll cuddle her.
(we're in a kind of long-distance relationship, so she's not over at my place that often.)
One time she was sick and had to stay at home from school, so I went out of my way during class to go on my phone and send her a ton of memes to cheer her up. (I have so many rare pepes saved on my phone just for her lmfao)