First of all sign into url as shown below
Enter your credentials to login into Office365 Project site, after successfully logging in you will able to see the below screen as a landing page.
Now click on Menu as shown below
Upon clicking on menu a list of items will be displayed as shown below
Select Project,
When you click on Project, you can see Project online landing page which is PWA (Project Web App) site as shown below.
Click on the settings gear as shown below
Select PWA Settings option from dropdown, then page navigates to PWA Settings landing page as shown below
Select Delete Enterprise Objects Option
Select the project you want to delete and click the Delete button as shown in the above Screenshot. The project will be deleted successfully.