Log in to the HTML5 client.
Select the Work workspace, Gantt tab.
Select a project in the Project Data Selector.
Select a task in the Gantt spreadsheet to display the Task panel.
Select the Transactions & Cost Items tab ().
In the Cost Item Assignment card, modify directly the Assignment Rate ().
OR, click on the More () button, and select the Edit command to open the Cost Item Assignment dialog box where the Rate can also be modified.