Removing iCloud on an old phone
Connect your iPhone 5c with your PC/Mac and open iTunes
  Restore the 5c to factory settings (http://support.apple.com/kb/HT1414)
  Go to your Apple Support Profile and de-register it.
  


That should be it.

In addition, you can delete old iPhone Backups from the iCloud:


  
  Go to Settings > iCloud > Storage &amp; Backup > Manage Storage.
  Tap the name of an iOS device whose backup you don’t need, then tap Delete Backup.