1. Unpair your Apple Watch if you have one
If you paired an Apple Watch with your iPhone, unpair your Apple Watch. Go to the Apple Watch app on your phone, make sure watch is with you. Go to the My Watch Tab and select your watch. Select the information icon next the watch you want to unpair. Select Unpair Apple Watch. Confirm (you may need to enter your Apple ID again to disable Activation Lock.) Before erasing your watch, your iPhone will create a backup of your Apple Watch, which you can use to restore your Apple Watch if you’re pairing it with a new iPhone.
2. Back up your iPhone
The simplest way is to use iCloud. At Settings > iCloud > Storage > Manage Storage, tap a device backup (such as your iPad or iPhone). If you like you can toggle off apps you don’t wont to have on your next phone. Then go to Settings > iCloud. If you want to back up your photos, toggle on Photos and then toggle on iCloud Photo Library. In Settings > iCloud make sure iCloud Backup is toggled on. Then Select Back Up Now. If you don’t have enough space in iCloud. you can purchase more space, i actually highly suggest this.
You can also backup to your computer using iTunes. Connect your device to your computer. Select your device when it appears in iTunes. If you want your photos backed up, make sure you turn that feature on. If you want to backup your Health and Activity data from your iPhone or Apple Watch, you’ll need to encrypt your backup by selecting the box Encrypt backup (make sure you create a password you can remember). The select Back Up Now
3. Turn off iMessage
Open Settings > Messages > iMessage > toggle it off.
4. Turn off Activation Lock
Go to Settings > iCloud and select Find My iPhone. Then toggle off Find My iPhone
5. Sign out of iCloud
Go to Settings > iCloud and select Sign Out. (If your phone is running iOS 7 or earlier, select Delete Account.) Select Sign Out again, then select Delete from My iPhone.
6. Sign out of the iTunes & App Store
Go to Settings > iTunes & App Store > Apple ID > Sign Out.
7. Restore to factory settings
Open the Settings app > General > Reset > Erase All Content and Settings. (If you turned on Find my iPhone, you may need to enter your Apple ID and passcode. You may also be asked for your Restrictions passcode.)
8. Call your carrier to remove your iPhone from your account
Call your carrier and ask them to remove your iPhone from your account.