Once you have changed your role to 'Controller' top right of your screen, you will find two sections: 'Expenses for controlling' and 'Expenses to approve'.
Choose a user in the first section and click on the name to see all his/her approved expenses.
If there are no 'special' expenses in the controlling dashboard, you do not have to open expense by expense. You can control all the expenses at once, by activating the checkbox (above all the expenses). All expenses will be selected and in the 'Actions'-button that appears you can now choose 'Control'. To customize your overview, get more info in Custom columns in expenses overviews.
Whenever you do want to take a closer look at an expense, you can easily find all the details by clicking on the expense. If your administrator has granted you permission to edit an expense, the 'Edit'-button will be visible for you in the detail of the expense.
When you reject an expense you will need to provide a reason. The user will see this reason and will be able to reply to it and resubmit the expense. The expense is now being sent back to the approver with an extra alert telling the approver that you as a controller rejected the expense. Or the user can delete the expense if it was indeed a mistake or against company policy.