how can i restore back my phone after factory reset

There are two ways based on where you had backed up your device.

Restore your device from an iCloud backup

1. Turn on your device. ... 
2. Follow the onscreen setup steps until you reach the Apps & Data screen, then tap Restore from iCloud Backup.
3. Sign in to iCloud with your Apple ID.
4. Choose a backup.

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Restore your iPhone, iPad, or iPod touch from a backup ...

Restore your device from a backup on your computer

1. On a Mac with macOS Catalina 10.15 or later, open Finder. On a Mac with macOS Mojave 10.14 or earlier, or on a PC, open iTunes.
2. Connect your device to your computer with a USB cable.* If a message asks for your device passcode or to Trust This Computer, follow the onscreen steps. If you forgot your passcode, get help.
3. Select your iPhone, iPad, or iPod touch when it appears in the Finder window or iTunes. If your device doesn't appear, learn what to do.
4. 
5. Select Restore Backup. If a message says that your device's software is too old, learn what to do. 
6. Look at the date of each backup and pick the most relevant.
7. Click Restore and wait for the restore time to finish. If asked, enter the password for your encrypted backup.
8. 
9. Keep your device connected after it restarts and wait for it to sync with your computer. You can disconnect after the sync finishes.
10. Get help restoring from a backup.