Vitamin C helps but don't use baking soda, it's an internet myth.
Pigmentation and tans usually go away on their own with time if you don't stay straight on the sun (aka. tan) every day.
Using good sunscreen on ur arms every day is probably the easiest way to get rid of it. I've never had an issue with tans going away and I've never used sunscreen but I'd imagine it makes the process much faster.