Forget the iPhone 5S, let's talk about the iPhone 6

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Sales of Apple's iPhone 5S have been impressive so far, but an iPhone 6 next year will likely give the company an even bigger boost, according to an analyst report.

(Read more: Apple's still got it: New iPhones ring up record sales)

Apple's iPhone 6 will likely be larger than the current models, featuring a display screen that is 4.8-inches, which will prompt more people who are available for an upgrade to purchase the device, said Peter Misek, an equity analyst at Jefferies, in a note. The latest models are 4-inches.

(Read more: Apple, China Mobile deal may be coming soon)

"Also the ecosystem is excited about the iPhone 6, which we feel is warranted. We think the stock will appreciate ahead of the iPhone 6 launch," he said in his note.

Misek, who recently returned from a trip to Asia where he met with Apple's suppliers, increased his price target for Apple from $425 to $600 and increased his rating for the stock to buy from hold.

(Read more: Why an iWatch may not be as big as the iPhone)

Upping Apple to buy

Previously, Misek feared Apple's gross margins would shrink because of increased costs from suppliers, but after his trip he said they have become more lenient on pricing.

"In contrast to earlier this year when suppliers boasted that Apple would not be able to push them around anymore, they are now scrambling to be in Apple's supply chain as competitor ramps have not gone as planned," he said in his note.

(Read more: How Apple has changed since Steve Jobs)

Misek estimates that there will be 85 million iPhones eligible for an upgrade by the time the iPhone 6 launches--which he expects to happen next September. In addition, the 5 million to 10 million customers who passed on the iPhone 5S, iPhone 5C cycle may upgrade at that time.

"A poor iPhone 5s/5c cycle means a bigger upgrade opportunity for the iPhone 6," he wrote. The larger screen and the estimated number of current Apple users expected to upgrade to an iPhone 6 next year will boost the company in 2015, he said.

However, Misek notes that while Apple's iPhone 5S remains in demand, the company is actually cutting orders for the less-expensive iPhone 5C because of slow sales. He estimates the company is cutting orders from 30 million to between 15 million and 20 million. And he said Apple may cut the price of the device to boost sales.

By CNBC's Cadie Thompson. Follow her on Twitter @CadieThompson.