How To Get White Underarms?

My underarm used to look good but then I had rashes. I kept on scratching which is why my underarm is now a bit dark. Prom is only 2 weeks away and It's really embarrassing if someone saw my underarm. I tried researching and most of them says that scrubbing lemon would help. I was just wondering if Calamansi ( citrus fruit - the small green one ) could be used as substitute? Lemons are quite expensive here so I just wanted to know if I can use them as substitute.

If you know other ways, I'd appreciate it if you tell me as well. Serious answers please. Thank you!