If you want to assign a fee schedule to all new projects of a client, you can set it up in Clients > General > Billing Options. This is applied to all new projects of the client. For existing projects of the client, you need to use the Batch Update option. To do that, filter the Projects screen by client to list the projects of the client that you want to update. Then use Actions > Batch Update. Check the Core Help for details.
You can also assign fee schedules to projects from the Fee Schedules screen. In the list view, select a fee schedule on the grid and click to select View Details. In the detail view, click Actions > Assign Project.