To edit the symbol text, you need to select the text, modify it as needed, then handle any resizing and flowchart re-layout issues.
To select the text, click on the interior of the symbol. You can select portions of the text using the mouse or the Shift and Arrow keys on the keyboard.
If you have the Select Objects tool enabled, you cannot select the text directly. Clicking anywhere on the symbol will select the symbol itself - not the text. You can either right-click on the symbol and select "Edit Text" from the popup menu, or you toggle the Select Objects tool off by clicking it again on the toolbar.
You can edit the text within the symbol just as you would with any text editor. Portions of the text can be selected and formatted with the Excel formatting controls available on the Home. This includes standard formatting functions like bold, italic, underline, and color formatting.
Note: Hitting the Enter key will add a line break within the symbol's text. When you are done editing the text, select anywhere else on the worksheet to exit editing mode.