To make changes to an existing project in Project Web App, you need to open it for editing.
To open a project for editing from the Project Center:
On the Quick Launch, click Projects.
Click the row header for the project you are opening in the list in the Project Center.
On the Projects tab, in the Project group, click the arrow on the Open button, and then click In Browser for Editing.
Note: When you use these steps to open a SharePoint task list project for editing, the task list opens and you can make changes there.
You can also open a project for editing from the Schedule page. The Schedule page is the page that appears when you click the name of a project in the Project Center.
To open a project for editing from the Schedule page, choose one of these options:
Click the arrow on the Edit button in the Project group on the Task tab, and then click In Browser.
Click Edit in the Project group on the Project tab.
Tip: You can also open a project for editing in Project Professional. For more information, see Open a project in Microsoft Project.