the specific element from the array in the dictionary?
var currency : [String: [Double]] = [
"CHF": [1.07, 1.06]
let eur_exchangeRate = currency["EUR"]
This will give you the corresponding array of doubles from the dictionary.
let buyRate = eur_exchangeRate!
This will give you the second element in the array of doubles (which, I believe is the buy rate you're wanting to access)
currency["EUR"]! = 99
This will set your buy rate for EUROS to 99.
Please look at @vadian's comment on your question to get an idea of a cleaner implementation of these sort.