If you want to add a line break in a table header, you can use the vbNewLine constant, which is provided by Visual Basic. For example, if you want to add a line break between the words Retention and Time, the expression in the table header will look like this:
="Retention" + vbNewLine + "Time"
This type of line break is not shown in the Editing Pane, but will be applied when you generate the report.