Warning: Greasemonkey is not supported. Use either Violentmonkey or Tampermonkey.
You can delete global history, or delete history for specific anime/manga:
to do this, you should open your anime/manga episode/chapter history first. You can do it in your profile history clicking on 'edit' link (open it in new tab by middle click) beside the entry you want to clear, or in Advanced Editing popup window ('Episode Details', or 'Chapter Details' URL).