Apple Inc. shares fell 10% Thursday after the iPhone maker slashed its quarterly revenue forecast for the first time in more than 15 years as it deals with shrinking demand in China and fewer owners upgrading their phones.

Chief Executive Tim Cook said in a letter to investors Wednesday that revenue was going to be about $84 billion for the quarter that ended late last month, a drop of at least $5 billion from the range of $89 billion to $93 billion Apple had been expecting.