How do I return my iPhone?

According to Apple:

Standard Return Policy

There are a few important things to keep in mind when returning a product you purchased online from Apple: 

• You have 14 calendar days to return an item from the date you received it.
• Only items that have been purchased directly from Apple, either online or at an Apple Retail Store, can be returned to Apple. Apple products purchased through other retailers must be returned in accordance with their respective returns and refunds policy.
• Please ensure that the item you're returning is repackaged with all the cords, adapters and documentation that were included when you received it.

There are some items, however, that are ineligible for return, including:

• Opened software*
• Electronic Software Downloads
• Software Up-to-Date Program Products (software upgrades)
• Apple Gift Cards (includes previous generation Apple Store Gift Cards and App Store & iTunes Gift Cards)
• Apple Developer products (membership, technical support incidents, WWDC tickets)
• Apple Print Products

*You can return software, provided that it has not been installed on any computer. Software that contains a printed software license may not be returned if the seal or sticker on the software media packaging is broken.

iPhone and iPad Returns — Wireless Service Cancellation

Wireless carriers have different service-cancellation policies. Returning your iPhone or iPad may not automatically cancel or reset your wireless account; you are responsible for your wireless service agreement and for any applicable fees associated with your wireless account. Please contact your provider for more information.

If it’s a month old you are past the return/exchange period. At this point your only options would be to have Apple help you with whatever issue you are having or selling your device(s) to recoup your money or pay off your installment if you financed.