Delete a site collection

1-Select the site collection you want to delete from the drop-down menu on the Delete Site Collection page

2-After selecting Change Site Collection from the drop-down menu, you will see a list of site collections displayed for the Web application that you have most recently accessed.

3-To display a list of site collections for a different Web application, use the Web Application drop-down menu located at the upper-right of the page

Note: When you delete a site collection, all the content is deleted within it, including subsites, lists, and libraries. Therefore, you should perform a backup of the content database containing the site collection or export the site collection first

4-After clicking OK on the Select Site Collection page, you will see the Delete Site Collection page displayed again with the entire path of the site collection shown along with a Delete button.

5-Click Delete and then click OK on the confirmation dialog box to delete the selected site collection.

If you have any queries/questions regarding the above mentioned information then please let me know, Thank you