Before select this option, please make sure you selected the right preset to edit.
Comparator Fast is "owner" of two names: Default and Last used settings. You're not allowed to "delete" those settings, must select any of the presets you saved.
You'll be asked for a confirmation to delete selected preset:
If you want to delete that preset (in this example was named "Tmp's"), hit the Ok button. You may abort this with a click on Cancel button.
Please note: once deleted, a preset is not recoverable. You'll need to create it again.