Typically you will export the entire pay period together, once timesheets have been approved. Sometimes though, you may need to just export one timesheet individually.
You might need to do this as you had already exported everyone else, but were waiting to approve this last person or you noticed once you had exported to your payroll that something wasn't right, so you fixed it up, deleted it and then need to re-export it.
To export just one timesheet, go to Timesheets and View Timesheets and click on the name of the relevant employee to go to their Timesheet:
Once the timesheet is approved, you will notice an Export to...button at the top:
Click this and select your relevant payroll system from the drop down. After this the Timesheet will show as being exported.
Just keep in mind, some payroll system such as Xero do pick up if you've already exported a Timesheet, so if you're missing one or two, you might think to just Export All again. For systems such as MYOB, you can export a timesheet as many times as you like, which will accumulate the hours in the employee's pay run.
It is safer to export a timesheet individually, as instructed above, than Export All again and risk duplicating anyone's pay.