The individual properties of a report element can be changed anytime during report design. Select the desired element and either double-click with the left mouse button, or press the right mouse button and choose "Properties" from the popup menu. The property dialog for the selected report element will open, which is the same dialog as the one that shows up when inserting a new report element. The following picture shows the property dialog for a label:
Edit the element properties to your needs and press the "OK" button to close the dialog and accept your changes. If you press "Cancel", all your changes are dropped and if you were inserting a new element, it will not be added to the report.
Information about the different properties for each report element can be found at the individual element descriptions.