Lead Lost Reasons can be managed in your company settings by navigating to settings-> Sales -> Lead Lost Reasons

Several actions can be performed on the lead lost list.

Add Row: Clicking add row will show a slide-out that will allow you to add an item to the list.

Edit: Clicking the three dots menu at the end of a row will allow you to edit the name of an existing item

Delete: Clicking the three dots menu at the end of a row will also allow you to delete the entry entirely.