iPhone 14 that's flashing green and won't turn off

Hello,

1. Press and quickly release the volume up button.
2. Press and quickly release the volume down button.
3. Press and hold the side button until you see the Apple logo (could be as long as 30 seconds) 

The timing is a little critical, you may have to try it several times. See this for more detail:

If your iPhone, iPad, or iPod touch won‘t turn on or is frozen

Also see:

What to do before you sell or give away your iPhone, iPad, or iPod touch

Good luck.