Bought a large wooden cutting board at Ikea and it’s stained...??

Hello, I got a large Ikea wooden cutting board for the kitchen that I had stored in the basement in its original packaging (shrink-wrapped). Our basement is not damp and we live in a new building! Now I brought it up after I now have quite a few things from VÄRDE in my kitchen *happy*... and what do I see?? The enclosed note in the packaging is wrinkled and yellowed and the worst part is that (I had the board standing upright on the floor in the basement) it must have gotten moisture at the bottom for almost 10 cm - there are dark spots?? What should I do? Should I have it cut at the hardware store? But then that side will definitely be very rough and nothing can be done about it with wood oil? Or should I sand it? But how do I do that properly? With which sandpaper (name/number etc.)? Sorry, but can you explain it to me step by step? I have never done anything like this... If I should sand it, I don’t know how deep the dark spots are or how much I need to sand off? Do I sand the whole board or only the spots, and do you always sand both sides? Do you think the spots will go away then? I don’t want my beautiful thick board to be only one centimeter thick afterwards haha*, just kidding aside... Do you think these strange spots are a health risk? The board was expensive and I really don’t want to just throw it away : ( Please help me : ( I thank you already for your help.