To make a CHANGE to a Workday entry, you need to be in the EDIT screen. Find your entry by searching for "My Expense Reports" or type of entry you created- you will see that you are in the VIEW screen -and in the red bar look for ACTIONS and click the 3 dots... after it- and choose CHANGE.
You should now see that you are in the EDIT screen and should have access to make the changes, even after it has been approved.