A new iPhone is expected to be released in the early fall. Users who have an old iPhone might want to think about selling it now when its value is still high, according to CNN Money.

The reason why iPhone users should start making plans to sell their old iPhones is because they might start dropping in resale value. The resale value of older iPhones start dropping by about 10 percent in September and October, according to eBay data analyzed by NextWorth, a website that accepts trade-ins of gadgets. September is usually when Apple releases an updated iPhone.

Waiting until the fall could greatly affect the trade-in value of an old iPhone because the supply will be abundant. By November and December the value of an older iPhone on the secondary market could decrease by up to 30 percent.

NextWorth's CMO Jeff Trachsel said they have seen the same patterns of decreasing iPhone prices for the past four years. NextWorth examined the sales of iPhones on eBay from July to December 2014, which amounted to hundreds of thousands of sales, Trachsel said.

"We fully expect this [pattern] to continue for the launch of Apple's upcoming phone," Trachsel said.

Many companies will accept trade-ins for old iPhones. The four main national carriers, Verizon, AT&T, Sprint and T-Mobile, will all purchase iPhones from customers. The Apple Store, Best Buy, Target, Wal-Mart and Amazon will as well. Trade-in websites like NextWorth and Gazelle will often offer higher values on old iPhones than major retailers. It is wise to shop around to get the best value for a retired iPhone.

NextWorth will purchase a 128 GB iPhone 6 that runs on the T-Mobile network for $380 if sold to them now. Waiting until November or December would decrease the value to $250.

An iPhone 6 Plus with 128 GB of storage and running on AT&T will currently sell to NextWorth for $435. If sold in December, that same iPhone would only sell for $287, NextWorth says.

The iPhones with less storage will also sell for a pretty penny, especially those on the Verizon network. A 16 GB iPhone 6 from Verizon will sell for $390. That same 16 GB iPhone 6 running on the Sprint network will only sell for $270.

To ensure the highest value for their old iPhones, customers will need to ensure they are in good, working condition. Additionally, companies will offer greater offers for phones that are unlocked.