The silver <—> gold exchange office moved from plus menu to the auction page. It’s only available after you have finished 10 adventures.
To get to the silver <—> gold exchange office click on the auction button next to your hero’s picture.
Here you can either exchange Gold into Silver (1 Gold = 100 Silver) or Silver into Gold (200 Silver = 1 Gold). Click on the button for Gold to Silver or Silver to Gold then type in the number of coins you want to exchange.