Source: Apple 
Today is the day to get your green on. It's Earth Day, and Apple is celebrating with a revised, iPad-friendly Reuse and Recycling program . The program accepts old, used iOS devices and offers Apple gift cards in return. Previously, old iPhones could only be traded in for new iPhone credit. Starting today, an old iPhone can be swapped for new iPad credit, or visa versa.
Source: Getty 
To trade in online, start by filling out the questionnaire , which will prompt you with questions about the functionality, condition, carrier, whether it has an engraving, and whether it has ever been water damaged. After completing the survey for a 16GB black iPhone 4s in good condition, the estimate return was $231, although CNBC  reports that the maximum trade-in value (the iPhone 5) is $336. Apple will then provide a shipping label and packaging materials and send a gift card within three weeks.
Think you could earn more by selling the used device on your own? Follow these tips  on how to prep your gadget for resale.
The program is all part of Apple's initiative to recycle responsibly all of its devices. Even if your device isn't worth trade-in credit, Apple stores will recycle all Apple products for free, and some retail locations hold special events where they collect other companies' electronics, too.