You get one or the other, but not both:
- Your new iPhone's data.
- Your old data.
So I will assume what you want is your old data, restored from the iCloud backup. In this case, knowing you will lose any data you have added to the new iPhone, simply do:
Settings > General > Reset > Erase All Content and Settings
... on the new iPhone. You may be prompted to turn off Find My iPhone, and perhaps other things. Follow these prompts, and once your iPhone is reset, you will be invited to restore the iCloud backup from your old iPhone (do not back up the new iPhone first!!)