Restoring with NightLight Desktop
If you've got NightLight Desktop then restoring the default icons is as simple as clicking the Default Icons tab and then "Install".
Manually Restoring the Default Icons
Restoring the default icons is (almost) just a case of deleting any custom ones that you've installed.
The first step is to locate where your game is installed - see Installation Path for info on how to do this.
Once you have the path, open it in File Explorer and navigate to
DeadByDaylight\Content\UI\Icons (if you didn't add it to the path already).
Inside that folder are various folders which correspond to the different icon types, i.e.
CharPortraits - delete whichever you are looking to reset to default.
Lastly, to help avoid any issues, you'll want to verify your game files.