To approve a Timesheet entry you must have "Approver" Timesheet user permissions. Here's how:
Click on the menu icon (the three lines) in the top left corner of the dashboard
Select Analytics from the menu
Scroll down to Detailed Reports and Select Timesheets.
Timesheets that need approving will sit under the Approvals tab. Locate the Timesheet entry that you want to edit and click the view button (blue eye) on the right-hand side the entry.
Mark the Timesheet as Approved by clicking the blue tick.
To view all Timesheets, including those already approved, click on the List View tab.