How do I restore my iphone from a previous back up?

Please follow the instructions in : https://support.apple.com/guide/icloud/restore-or-set-up-your-ios-or-ipados-device-mm7e756df7fd/icloud

Assuming you have a Mac, connect your phone to the Mac using the USB cable, open Finder, locate your phone in the left pane and click Restore Backup. You will have the ability to choose which backup to use.