how do i do a hard reset on my iphone

A hard reset on an iPhone usually means a force restart , not a factory reset. It does not erase your data. Apple’s current instructions say to press and quickly release Volume Up, press and quickly release Volume Down, then press and hold the Side button until the Apple logo appears.

For iPhone 8 and later

1. Press and quickly release Volume Up.
2. Press and quickly release Volume Down.
3. Press and hold the Side button.
4. Release when you see the Apple logo.

For iPhone 7 and 7 Plus

1. Press and hold Volume Down and the Side button together.
2. Release when the Apple logo appears.

For iPhone 6s and earlier

1. Press and hold the Home button and the Top/Side button together.
2. Release when the Apple logo appears.

If you meant factory reset

That is different from a hard reset. To erase the phone, go to Settings > General > Transfer or Reset iPhone > Erase All Content and Settings.

Quick note

If your iPhone is frozen, a force restart is the first thing to try; if you want to sell it or wipe it clean, use the erase option instead.