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.