How do I restore my iPhone?

If you have an iCloud Backup, or a Backup on your computer from that day, you can restore to it. However, you must be aware that if you do this, your phone will be overwritten with that backup - so what you have on your iPhone now will disappear.

Remember that iCloud Backup does not contain content that is already backed up in iCloud. E.g. Notes, Pictures, Messages, Calendar, etc. Backups made on a computer contain everything.

You can see how to restore from a backup here:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Karim997 Said:

"How do I restore my phone to what it was a few days ago?"

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Downgrading the iOS:

Did you just install an update? If so, then note that ihe iOS cannot be downgraded.