Hi PtonTri,
Thanks for using Apple Support Communities. We're assuming that you're referencing your iPhone 7. Can you provide more details about exactly what it is that you're experiencing with iOS14? In the meantime, does restarting your device temporarily address the issue your facing? See: Restart your iPhone - Apple Support
How to restart your iPhone SE (2nd or 3rd generation), 8, 7, or 6
1. Press and hold the side button until the power-off slider appears.

2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button until you see the Apple logo.
Keep us posted on the results.
Cheers.