@jayv25585659 : There is no option to delete it once you have provided a thumbnail. However, you can update it to restore to previously used thumbnail by below options I assume:-
Select Image - You can select the original thumbnail by choosing this option and select the original thumbnail image from DAM.
Upload Image - You could upload the original thumbnail by choosing "Upload Image".
If you were in middle of making changes and not happy with what you have selected, you see a 'Revert' option to un-do the changes, but that is until you have not saved your changes.
Please refer this to see the options to configure page thumbnail:-
https://experienceleague.adobe.com/docs/experience-manager-cloud-service/content/sites/authoring/fundamentals/page-properties.html?lang=en#images